air brakes manual
TRANSCRIPT
-
7/29/2019 Air Brakes Manual
1/95
Air Brake ManualWorking with driversto make our roads safer.
-
7/29/2019 Air Brakes Manual
2/95
Air Brake Endorseme Permitstheholdertodrivevehicles
equippedwithairbrakesinclassof
vehicleforwhichthedriverislicenc
Toadjustmanualorautomaticslack
adjusters,theoperatormustholdan
brakeendorsement.
Requirements for AiBrake Endorsement MustcompleteanAirBrakeWritten
MustcompleteanAirBrakePractica
Anairbrakeendorsementisnotrequi
foralicensedholdertodriveaClass3airbrakeequippedvehicleifitisregis
asafarmvehicle.Class3or5airbrake
equippedvehicles,fittedwithdealer
plates,maybedrivenbyfarmersfor
demonstrationpurposes.
Use of Licence asa Learner PermitAholderofanylicencemaybeauthor
tooperatevehiclesequippedwithairb
asalearneraftermeetingtherequired
writtenstandards.Thelearnermustb
supervisedbysomeonewhohasheld,
atleasttwoyears,anairbrakeendors
ForewordheAirBrakeManualhasbeenprepared
yManitobaPublicInsurance,toassist
riversinunderstandingthebasicoperation
ndfunctionofanairbrakesystem.
hestudyofthismanual,togetherwith
racticalinstruction,isrecommended
oradriverwhoispreparingfortheair
rakeexamination.Alargeillustrationof
completedualairbrakesystemislocated
ntheinsidecoverandcanbefoldedout
ndreferredtowhenstudyingthismanual.
tudyquestionsareincludedattheendof
achsectionsothatreadersmayselftest
heirunderstandingofthesubjectmatter.
riverswhohavequalifiedandareauthorized
ooperateairbrakeequippedvehiclesarencouragedtoreviewthismanualona
eriodicbasistoensuretheyarefully
wareofthepropermethodofinspecting
nairbrakesystemandidentifyingproblems
hatcanoccurwhenthesystemmalfunctions.
heillustrationsandexplanationsofvarious
ypesofbrakesystemdesignsareprovided
orinstructionalpurposesonly.Mostair
augesmeasureinimperialunits.Therefore
hemeasurementsusedandrelatingtothe
rbrakesystemwillbeinimperialunits.
hismanualhasnolegislativesanction.
orinterpretingandapplyingthelaw,
onsultThe Drivers and Vehicles Act,The Highway
raffic Actanditsregulations.Salesofthis
manualarefinalandnotrefundable.
Wegratefullyacknowledgethecontributionsof
ljurisdictions,particularlyBritishColumbia.
-
7/29/2019 Air Brakes Manual
3/95
Dual Air Brake System
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16
17
6
18 15141920212211 122324252628
2729
33
30
6 6
1. Compressor
2. Governor
3. Air dryer
4. Safety valve
5. Supply/wet reservoir
6. Drain valves7. Oneway check valves
8. Primary/dry reservoir
9. Low pressure indicator
10. Secondary/dry reservoir
11. Rear service brake chambers
12. Spring parking brake chambers
13. Tractor relay valves
14. Trailer service brake chamber
15. Trailer spring parking brake chamber
16. Trailer reservoirs
17. Trailer relay valve
18. Trailer spring brake valve
19. Anticompound lines
20. Glad hands
21. Supply (emergency) line
22. Control (service) line
23. Spring brake modulator v
24. Tractor protection valve
25. Stop lamp switch
26. Twoway check valves27. Spring parking brake cont
28. Trailer supply valve
29. Reservoir air pressure gau
30. Trailer brake hand valve
31. Foot valve
32. Front service brake cham
33. Quick release valve
34. Automatic front brake lim
This illustration has an autom
brake limiting valve (34), and t
the control valve (35), for a ma
brake limiting valve (36) are no
here, but appear later in the m
Legend
bluesupply/
greenprima
redsecondar
yellowspring
brake system
dark greentr
Incab portiohighlighted
-
7/29/2019 Air Brakes Manual
4/95
CHECK THE SLACK!
It is up to YOU,the DRIVER,
to ensure that
your vehicle hassafe, properly
adjusted brakes.
-
7/29/2019 Air Brakes Manual
5/95
Air Brake Manual
Table of ContentsForeword Foldot
AirBrakeEndorsement Foldot
RequirementsforAirBrakeEndorsement Foldot
UseofLicenceasaLearnerPermit Foldot
DualAirBrakeSystemIllustrationFoldot
HowtoBookYourTestAppointment 3
KeyPointsAboutYourTests 4
Section 1 Brakes and Braking 5
HeatEnergyTractionFriction 6
SpeedWeightDistance 7
BrakingForce 7
UseofAirPressure 8
LeverageandAirPressure 8
StoppingDistance 8
SectionSummaryQuestions 9
Section 2 The Components of an Air Brake System 11
TheComponentsofanAirBrakeSystem 12CompressorandGovernor 12
Reservoirs 14
AirDryer 15
SafetyValve 16
FootValve 16
BrakeChambers,SlackAdjustersandBrakeLining 16
WedgeBrakes 20
DiscBrakes 21
AirOverHydraulicBrakeSystems 21
AirActuatedHydraulicBrakeSystem 21
AirBoostHydraulicBrakeSystem 22
SectionSummaryQuestions 24
Section 3 How the Basic System Works 25
BasicAirBrakeSystem 26
OnewayCheckValve 26
AirPressureGauge 27
BrakeApplicationGauge 27
LowPressureWarningDevice 27
StopLightSwitch 27
QuickReleaseValve 28
RelayValve 28
ManualFrontBrakeLimitingValve 28
AutomaticFrontBrakeLimitingValve 29
TandemRearAxles 30
SectionSummaryQuestions 30
Section 4 Spring Parking Brakes
SpringParkingBrakeSystems
UsingaSpringParkingBrake
MechanicalRelease(Caging)
SectionSummaryQuestionsSection 5 Trailer System
GladHands
ApplicationLine
TrailerBrakeHandValve
TwowayCheckValves 4
TractorProtectionSystem
TractorProtectionValve
TrailerSupplyValve
AutomaticTrailerSupplyValveSystem 4
TractorandTrailerCoupled 4
ChargingtheTrailerSystem
FootorHandValveBrakeApplicationEmergencyApplication 4
Supply(Emergency)LineRupture
Control(Service)LineRupture
LossofReservoirAirPressure
ManualTrailerSupplyValve
TrailerSpringParkingBrakes
SectionSummaryQuestions
Section 6 Dual Air Brake System
BasicDualAirBrakeSystem
DualAirBrakeSystemwithSpringParkingBrakes
SpringParkingBrakeswithModulatorValve
CombinationTractorandTrailer withSpringParkingBrakes
SectionSummaryQuestions
Section 7 Electronic Controlled Brakingand Traction Systems 6
AntilockBrakeSystem(ABS)
AutomaticTractionControl(ATC)
SectionSummaryQuestions
-
7/29/2019 Air Brakes Manual
6/95
2 Air Brake Manual
Section 8 Brake Adjustment and In-Service Check 65
BrakeAdjustment 66
BrakeAdjustmentItsCritical 66
BrakeAdjustmentItstheLaw 66
Definition 67
CheckingandAdjustingSCamBrakes
ManualSlackAdjusters 69PryMethodofFreeStrokeMeasurement 69
AppliedStrokeMethod(servicebrakeapplication) 70
BrakeAdjustmentIndicators 70
BrakeAdjustmentManualSlackAdjuster 71
Adjustment 71
AdjustingS-CamBrakeswithAutomaticSlack
AdjustersWhyWhenHow 72
EmergencyManualBrakeAdjustment
AutomaticSlackAdjusters 73
DiscBrakeAdjustment 75
WedgeBrakeAdjustment 75
AfteraBrakeAdjustment 75
In-ServiceChecks 76MaintenanceandServicingofthe
AirBrakeSystem 76
AirBrakeAdjustmentMyths 77
SectionSummaryQuestions 78
Section 9 PreTrip Air Brake Inspection 79
VehicleInspection 80
AirBrakePracticalTest 80
PreTripInspectionCombinationUnit 81
BrakeAdjustmentCombinationUnit 82
PreTripInspectionSingleUnit 85
BrakeAdjustmentSingleUnit 86PreTripInspectionAirOverHydraulic
(AirActuated)BrakeSystem 88
SectionSummaryQuestions 89
MetricConversionTable 89
Pre-tripInspectionTearOutSheet 91
OrganandTissueDonation 93
-
7/29/2019 Air Brakes Manual
7/95
Air Brake Manual
Test Appointments
How to Book Your Knowledge or Road Test
Visit any Autopac agent* to pay for your test and to
schedule your test appointment.
The agent can provide you details of available
appointments at any testing location.
You can schedule your test appointment up to eight
weeks in advance.
Both knowledge and road tests are conducted during
weekdays only (with the exception of holidays) at most
locations. Saturday appointments are available in
Winnipeg at Barnes Street, Lexington Park and Main
Street locations.
Test Locations Winnipeg
15 Barnes Street (at Bison Drive)
40 Lexington Park (at Gateway Road)
1284 Main Street
930 St. Marys Road (French Language Services available)
125 King Edward Street East
1103 Pacic Avenue
420 Pembina Highway
Test Hours
BarnesStreet,LexingtonParkandMainStreet:
MondayFriday7:30a.m.5:30p.m.
Saturday8:30a.m.4p.m.
Allotherlocations:
MondayThursday7:30a.m.5:30p.m.
Friday7:30a.m.4:30p.m.
(Closeat4:30p.m.MondayFridayduringJune,
JulyandAugust.)
Test Locations Outside Winnipeg
Arborg|323SunsetBoulevard
Beausejour|848ParkAvenue
Brandon|7311stStreet
Dauphin|217IndustrialRoad
Portage la Prairie|25TupperStreet
Selkirk|1008ManitobaAvenue
Steinbach|165ParkRoadWest,Unit2,
ClearspringVillageMall
The Pas|424FischerAvenue
Thompson|53CommercialPlace
Winkler|355BoundaryTrail
Test Hours
Alllocations:
MondayFriday8:30a.m.4:30p.m.
Mobile Test Units
Mobile Test Units serve rural customers living outside
the above locations. Mobile Test Unit locations and
schedules are available from any Autopac agent.
*For a list of Autopac agents, see mpi.mb.ca.
Key Points About Your Tests
Cost*
Aknowledgetestcosts$10.
Costsforroadtests:
Class1licence:$50 Class5licence:$30 Class2or3licence:$45 Class6licence:$30
Class4licence:$35 AirBrake:$30**
*Costmaychange
**Thereisnoadditionalchargefortheairbraketest
ifyouaretakingaroadtestforaclass1,2,3,4or5
licenceandthevehicleyouareusingforthetestis
equippedwithairbrakes
General Testing Information
Thereisa30-minutetimelimittocompletea
knowledgetest.
Onlyoneknowledgeorroadtestofthesameclassmbecompletedperday.
Thereisawaitingperiodbetweentheknowledgeand
roadtestforsomeclassesoflicences.
IfyouareintheGraduatedDriverLicensing(GDL)
programandarenotsuccessfulonyourknowledge
test,thereisaseven-daywaitingperiodbeforeyou
maytakethetestagain.
IfyouareinGDLprogramandarenotsuccessfulon
yourroadtest,thereisa14-daywaitingperiodbefore
youmaytakethetestagain.
Theknowledgetestisnotanopenbooktest.Cellularphonesandelectronicdevicesarenotallowedinthe
testarea.
Pleasecheck-in15minutespriortoyourscheduled
appointmenttime.
Once youve made your test appointment at anAutopac agent, you can call your agent or the followinnumbers for additional information or assistance:
InWinnipegcall2049857000
OutsideWinnipegcall18006652410
-
7/29/2019 Air Brakes Manual
8/95
Air Brake Manual
BrakesandBraking
Section1
-
7/29/2019 Air Brakes Manual
9/95
6 Air Brake Manual
HeatEnergyTractionFriction
Foravehicletomovealongthehighway,aninternal
combustionenginemustconvertitsheatenergyinto
mechanicalenergy.Thismechanicalenergygoes
fromtheenginetothedrivingwheeltiresbymeans
ofasystemofconnectingrods,shaftsandgears.
Thefinalfactorthatmovesthevehicleistheamountoftractionitstireshaveontheroadsurface.
Frictionistheforcethatresistsmovementbetweentwo
surfacesincontactwitheachother.Tostopavehicle,
thebrakeshoeliningsareforcedagainstthemachined
surfacesofthebrakedrums,creatingfriction.
Thisfrictionproducesheat.
Theengineconvertstheenergyofheatintotheenergy
ofmotion;thebrakesmustconvertthisenergyof
motionbackintotheenergyofheat.Thefriction
betweenbrakedrumsandliningsgeneratesheatwhile
reducingthemechanicalenergyoftherevolvingbrake
drumsandwheels.Theheatproducedisabsorbedby
themetalbrakedrums,whichdissipatetheheatinto
theatmosphere.Theamountofheatthebrakedrums
canabsorbdependsonthethicknessofthemetal.
Whenenoughfrictioniscreatedbetweenthebrake
liningandthedrums,thewheelsstopturning.
Thefinalfactorthatstopsthevehicleisthetraction
betweenthetiresandtheroadsurface.
Ifa200horsepowerengineacceleratesavehicle
to100km/hinoneminute,imaginethepower
neededtostopthissamevehicle.Also,considerthat
thevehiclemighthavetostopinanemergencyin
aslittleassixseconds(just1/!)thetimeittook
toreach100km/h).
Tostopthevehiclein1/!)thetimeittookto
acceleratewouldrequireastoppingforceof10
timestheaccelerationforcetheequivalentofapproximately2,000horsepower.Ifthevehiclehad
sixwheels,eachwheelwouldhavetoprovide1/^the
brakingforce.Ifoneortwoofthewheelshadbrakes
thatwerenotproperlyadjusted,theotherwheels
wouldhavetodomorethantheirshareofthebraking,
andthatmightbemorethantheirbrakeswere
constructedtostand.Excessiveuseofthebrakeswould
thenresultinabuildupofheatgreaterthanthebrake
drumscouldabsorbanddissipate.Toomuchheat
resultsinbrakedamageandpossiblefailure.
Mostbrakeliningsoperatebestataround250Cand
shouldnotexceed425C.Itsimportanttounderstandthatthepowerneededtostopgeneratesheatwhich
coulddamagethebrakes.
100 km/h
10X
Brake Drums
250CNormal
1100CPanic!
425CMaximum
-
7/29/2019 Air Brakes Manual
10/95
Air Brake Manual
SpeedWeightDistance
Thedistancerequiredtostopavehicledepends
onitsspeedandweight,inadditiontoenergy,
heatandfriction.Thebrakingforcerequiredto
stopavehiclevariesdirectlywithitsweightand
speed.Forexample,iftheweightisdoubled,the
brakingforcemustbedoubledtobeabletostopinthesamedistance.Ifthespeedisdoubled,
thebrakingforcemustbeincreasedfourtimes
tobeabletostopinthesamedistance.When
weightandspeedarebothdoubled,thebraking
forcemustbeincreasedeighttimestobeableto
stopinthesamedistance.
Forexample,avehiclecarryingaloadof14,000kg
at16km/hisbroughttoastopin30metreswith
normalapplicationofthebrakes.Ifthissame
vehiclecarried28,000kgat32km/h,itwould
requireeighttimesthebrakingforcetostopthe
vehiclein30metres.Thiswouldbemorebraking
forcethanthebrakescouldprovide.Novehiclehas
enoughbrakingforcewhenitexceedsitslimitations.
Braking Force
Mechanical
Brakingsystemsusedevicestogainamechanical
advantage.Themostcommondeviceforthis
purposeisleverage.Aleverisplacedonapivot
calledthefulcrum.AsthedistancefromAtoC
isfourfeet,andfromCtoBisonefoot,theratio
isfourtoone(4:1).Forcehasbeenmultipliedby
theleverageprinciple.
Lookatthissimpleleversystem:
Ifa100lbdownwardforceisappliedatpointA,then
theupwardforceatpointBis400lb.
B = 400 lb
C
A = 100 lb
C
A100 lb
Scam brake
B400 lb
B400 lb
A B
4 eet 1 oot
C
Applied orce= 100 lb
Delivered o= 400 lb
A
BC
-
7/29/2019 Air Brakes Manual
11/95
8 Air Brake Manual
Use o Air Pressure
Forcecanalsobemultipliedbytheuseofairto
gainfurthermechanicaladvantage.Everyonehas
felttheforceofaironawindyday.Aircanbe
compressed(squeezed)intoamuchsmallerspace
thanitnormallywouldoccupy,forinstance,
aircompressedintirestosupporttheweightofavehicle.Thesmallerthespaceintowhichairis
squeezed,thegreatertheairsresistancetobeing
squeezed.Thisresistancecreatespressure,which
isusedtogainmechanicaladvantage.
Ifaconstantsupplyofcompressedairisdirected
throughapipethatisoneinchsquare,andifa
oneinchsquareplugwereplacedinthepipe,
thecompressedairwouldpushagainsttheplug.
Ascalecanbeusedtomeasurehowmanypounds
offorcearebeingexertedbytheairagainsttheplug.
Ifthescaleregisters10pounds,forexample,then
itcouldbesaidtheforceis10poundsontheone
squareinchsurfaceoftheplugor10poundsper
squareinch(psi).
Themorecompressedtheairinthesupplyreservoir,
thegreatertheforceexertedonthefaceoftheplug.
Leverage and Air Pressure
Inactualoperation,pipesareroundandplugsare
diaphragmsofflexiblematerialactingagainstpush
rods.Ifcompressedairof120psiactsonadiaphragm
of30squareinches,3,600lbofforceisproduced
(120x30).Applythisforcetoapushrodtomove
a6inchslackadjusteroperatingacamandthetotalforceequals21,600inchpoundstorque(3,600x6),
or1,800footpoundstorque(21,60012).Itrequires
25to30footpoundsoftorquetotightenthewheel
onacar.Thiscomparisonillustratestheforce
obtainedfromusingmechanicalleverageandair
pressurecombined.
Stopping Distance
Stoppingdistanceconsistsofthreefactors:
driversreactiontime
brakelag
brakingdistance
Driversreactiontime:Reactiontimeisoften
calledthinkingtime.Thetimeittakesfrom
themomentahazardisrecognizedtothetime
thebrakeisapplied,approximately Pofasecond.
Brakelag:Asairishighlycompressible,itrequires
arelativelylargevolumeofairtobetransmitted
fromthereservoirtothebrakechamberbefore
thereisenoughpressureforthebrakestoapply.
Itcanbesaidthatbrakelagisthetimeittakes
theairtotravelthroughaproperlymaintained
airbrakesystem(approximately 4/!)ofasecond).
1 squareinch
10 psi
30 square inches
120 psi
6 inches
1inch
-
7/29/2019 Air Brakes Manual
12/95
Air Brake Manual
Passengercar
Brakes applied
Loadedtruck
Actual stop
Actual stop
Brakingdistance:Theactualdistancethevehicle
travelsafterthebrakeisapplieduntilthevehiclestops.
Thedistancedependsontheabilityofthebrakelining
toproducefriction,thebrakedrumstodissipateheat
andthetirestogriptheroad.
Driversshouldnevertaketheirbrakesforgranted.
Thebrakingsystemmustbetestedandtheadjustmentcheckedbeforeplacingthevehicleintoservice.Drivers
mustunderstandthebrakingsystem,realizeits
capabilitiesandlimitations,andlearntousethem
tothebestadvantage.
Heavyvehiclesrequirepowerfulbrakingsystems
thatareobtainedbyuseofmechanicalleverageand
airpressure.Brakesmustbeusedkeepinginmind
theheatgeneratedbyfriction.Iftheheatbecomes
toogreat,brakingeffectivenesswillbelost.Theheavier
theloadandthefasterthespeed,thegreatertheforce
neededtostop.
Itisimportanttorememberthatanairbrakeequippedvehicle,evenwithproperlyadjustedbrakes,willnot
stopasquicklyasapassengercar.
Comparative Stopping Distances
Section Summary Questions
1. Whatisthefinalfactorthatwilldetermine
ifthevehiclewillmove?
2. Whatisthefinalfactorthatwilldetermine
ifthevehiclewillstop?
3. Howistheheatthatisgeneratedbythebrakesdissipated?
4. Ifonesetofbrakeshoesispoorlyadjusted,
whateffectcouldithaveontheremaining
setsofbrakeshoesinthesystem?
5. Whatismeantbythetermfriction?
6. Iftheweightofthevehicleisdoubled,howmany
timesmustthestoppingpowerbeincreased?
7. Ifthespeedofthevehicleisdoubled,howmany
timesmustthestoppingpowerbeincreased
tobeabletostopatthesamedistance?
8. Ifbothweightandspeedofthevehiclearedoubled,howmanytimesmustthestopping
powerbeincreasedtostopatthesamedistance?
9. Whatiscompressedair?
10. Whatdoestheabbreviationpsistandfor?
11.If40psiisexertedagainstadiaphragmof
30squareinchesinarea,whatarethetotal
poundsofforcethatcouldbeexerted?
12.Stoppingdistanceconsistsofwhatthreefactors?
13.Definethefollowingterms?
DriversReactionTime
BrakingDistance
BrakeLag
-
7/29/2019 Air Brakes Manual
13/95
Air Brake Manual
TheComponentsofanAirBrakeSystem
Section2
-
7/29/2019 Air Brakes Manual
14/95
12 Air Brake Manual
SectionOneofthismanualhasexplainedthatitis
possibletogainamechanicaladvantagethroughthe
useofleversandthatairunderpressurecanbeused
togainamechanicaladvantage.SectionTwowill
explainhowairunderpressurecanbeusedto
operatetheairbrakesofavehicle.
Pipingillustrationshavebeenkeptsimpleinordertobeeasilyunderstood.Thepipingarrangementsfound
onvehiclesinactualuseonthehighwaymightdiffer
somewhatfromtheillustrationsinthismanual.
The Components o anAir Brake System
Abasicairbrakesystemcapableofstopping
avehiclehasfivemaincomponents:
1.Acompressortopumpairwithagovernor
tocontrolit.
2.Areservoirortanktostorethecompressedair.
3.Afootvalvetoregulatetheflowofcompressedair
fromthereservoirwhenitisneededforbraking.
4.Brakechambersandslackadjusterstotransfer
theforceexertedbythecompressedairto
mechanicallinkages.
5.Brakeliningsanddrumsorrotorstocreate
thefrictionrequiredtostopthewheels.
Itisnecessarytounderstandhoweachofthese
componentsworkbeforestudyingtheirfunctions
intheairbrakesystem.
Compressor and Governor
Compressedairisusedtotransmitforceinanair
brakesystem.Thesourceofthecompressedairis
acompressor(1).Acompressorisdesignedtopump
airintoareservoirwhichresultsinpressurizedair.
Thecompressorisdrivenbythevehiclesengine,eitherbybeltsandpulleysorshaftsandgears.Invehicles
wherethecompressorisdrivenbybelts,theyshouldbe
checkedregularlyforcracksandtension.Also,checkthe
compressorforbrokenmountingbracketsorloosebolts.
Thecompressorisinconstantdrivewiththeengine.
Whenevertheengineisrunning,soisthecompressor.
Whenpressureinthesystemisadequate,anywhere
fromalowof80psitoahighof135psiitisnot
necessaryforthecompressortopumpair.Agovernor
(2)controlstheminimumandmaximumairpressure
inthesystembycontrollingwhenthecompressor
pumpsair.Thisisknownastheloadingor
unloadingstage.Mostcompressorshavetwo
cylinderssimilartoanenginescylinders.When
thesystempressurereachesitsmaximum,which
isbetween115and135psi,thegovernorplacesthe
compressorintheunloadingstage.
Thecompressormustbeabletobuildreservoirair
pressurefrom50to90psiwithinthreeminutes.
Ifunabletodosothecompressorrequiresservicing.
Acompressormaynotbeabletobuildairpressure
from50to90psiwithinthreeminutesiftheair
filterispluggedorifthebeltwasslipping,ifthese
werenotatfaultthecompressorcouldbefaulty.
Governor
Pressuresettingspring
Exhaust port
Unload port
Reservoir port
Exhaust port
Unload port
Reservoir port
-
7/29/2019 Air Brakes Manual
15/95
Air Brake Manual
Placingthecompressorintheunloadingstageisdone
bydirectingairpressuretotheinletvalvesofthe
compressor,holdingthemopen,allowingtheairto
bepumpedbackandforthbetweenthetwocylinders,
insteadofcompressingtheair.Whenthepressure
inthesystemdrops,theinletvalvesclose,returning
thecompressortotheloadingstage.Thegovernor
mustplacethecompressorintheloadingstageat
nolowerthan80psi.Duringtheunloadingstage,
thecompressorisabletocool.
Usuallycompressorsarelubricatedfromtheengine
lubricationsystem,althoughsomecompressorsare
selflubricatingandrequireregularchecksofthe
lubricantlevel.
Itisveryimportanttheairthatentersthesystem
bekeptascleanaspossible.Theairmustfirstpass
throughafiltertoremoveanydustparticles.Theair
filtermustbecleanedregularly.Adirtyfilterwill
restricttheflowofairintothecompressor,reducing
itsefficiency.Somevehicleshavetheinletportof
thecompressorconnectedtotheintakemanifold
andreceiveairthathasbeenfilteredbytheengine
aircleaner.
Apistontypecompressoroperatesonthesame
principleastheintakeandcompressionstrokes
ofanengine.
Intakestroke:Thedownwardstrokeofthepistoncreatesavacuumwithinthecylinderwhichcauses
theinletvalvetoopen.Thiscausesatmospheric
airtoflowpasttheinletvalveintothecylinder.
Piston
From governor
Intake air ilter
Unloadplunger
Inlet valve
Discharge valve
Compressor (Unloading stage)
Intake air ilter
Unloadplunger
Inlet valve
Discharge valve
Compressor (Intake stroke)
Piston
-
7/29/2019 Air Brakes Manual
16/95
14 Air Brake Manual
Compressionstroke:Theupwardmotionofthepistoncompressestheairinthecylinder.Therising
pressurecannotescapepasttheinletvalve(which
thecompressedairhasclosed).Asthepistonnears
thetopofthestroke,thepressurizedairisforced
pastthedischargevalveandintothedischargeline
leadingtothereservoir.
Reservoirs
Reservoirsortanksholdasupplyofcompressedair.
Thenumberandsizeofthereservoirsonavehicle
willdependonthenumberofbrakechambersand
theirsize,alongwiththeparkingbrakeconfiguration.
Airbrakevehiclesareequippedwithmorethanone
reservoir.Thisgivesthesystemalargervolumeofmain
reservoirair.Thefirstreservoirafterthecompressoris
referredtoasthesupplyorwet(5)reservoir.Theother
reservoirsareknownasprimary(8)andsecondary(10)
ordry(8)(10)reservoirs.Whenairiscompressed,
itbecomeshot.Theheatedaircoolsinthereservoir,
formingcondensation.Itisinthisreservoirthat
mostofthewateriscondensedfromtheincomingair.
Ifoilleakspastthepistonringsofthecompressor
andmixeswiththismoisture,itformssludge,which
accumulatesinthebottomofthereservoir.Ifallowed
toaccumulate,thissludge(waterandoil)wouldenter
thebrakingsystemandcouldcausetroublewithvalves
andotherparts.Inwinter,waterinthesystemmay
freeze,causingthemalfunctionofvalvesorbrake
chambers.Reservoirsareequippedwithdrainvalvesso
thatanymoistureorsludgethatmayhaveaccumulated
canbedrained.Ifyounoticesludgewhendrainingyour
system,haveitinspectedbyamechanic.Tominimize
theamountofwatercollection,allreservoirsmust
bedraineddaily.Underextremeconditions,reservoirs
mayhavetobedrainedmorethanonceaday.Todrain
thereservoirsalwaysstartwiththewetreservoiron
thetractor.Openthedrainvalvefullyandallowall
airpressuretoescape,whichwillalsoexhaustthe
moisturecollectedinthereservoir.
Somereservoirshavemorethanonecompartment
andeachcompartmenthasitsowndrainvalve,
whichmustbedrainedindividually.Brieflyopening
thevalvejusttoallowsomeoftheairtoescapedoesnotdrainthemoisture!Itisnotsafetoassumethat
thewetreservoir,orthepresenceofanairdryeris
reasontoneglecttheotherreservoirsonthepower
unit,trailersordollies.Theyshouldallbecompletely
draineddailybyopeningthedrainvalvefullyand
allowingalloftheairtoescape.Thisshouldbedoneat
theendoftheday.
Somereservoirsmaybeequippedwithautomatic
reservoirdrainvalves(spittervalves).Thesevalves
willautomaticallyexhaustmoisturefromthe
reservoirwhenrequired,althoughtheyshould
becheckeddailyanddrainedperiodicallytoensurethemechanismisfunctioningproperly.Anyloose
ordisconnectedwiresassociatedwiththevalve
heatersshouldberepairedimmediately.
Reservoir
Intake air ilter
Unloadplunger
To reservoir
Inlet valve
Discharge valve
Compressor (Compression stroke)
Piston
-
7/29/2019 Air Brakes Manual
17/95
Air Brake Manual
Air Dryer
Anairdryer(3)maybeinstalledbetweenthe
compressorandthewetreservoirtohelpremove
moisturefromthecompressedair.Itmaybepartially
filledwithahighmoistureabsorbentdesiccantand
anoilfilter,oritmaybehollowwithbafflesdesigned
toassistinseparatingthemoisturefromtheair.Both
typesofairdryersuseairpressuretopurgeorejectthe
accumulatedcontaminantsfromtheirdesiccantbed.
Thepurgevalvehasaheaterelement,whichprevents
themoisturefromfreezingincoldclimateoperation.
Thewiringconnectedtotheheatershouldbeinspecte
forlooseordisconnectedwires.Theyarealsoequipped
withasafetyvalve.
Controlport
Supplyport
Cutopiston
Exhaust
Purge
valve
Delivery port
Oneway check valve
Oneway check valve
Oriice
Desiccant bed
Control PortDried Ai
Check valvassembly
Delivery Port
Heaterelement
ExhaustPurgevalve
Cutopiston
SupplyPort
Reservoir
Compressor
GovernorSump
Air Dryer (Purge cycle)
DesiccaCartridg
Air Dryer (Drying cycle)
Air Dryer
Control Port
Supply Port
Oil Separator
-
7/29/2019 Air Brakes Manual
18/95
16 Air Brake Manual
Saety Valve
Asafetyvalve(4)protectsreservoirsfrombecoming
overpressurizedandburstingifthegovernor
malfunctionedanddidnotplacethecompressor
intheunloadingstage.Thevalveconsistsofa
springloadedballthatwillallowairtoexhaust
fromthereservoirintotheatmosphere.Thevalves
pressuresettingisdeterminedbytheforceofthe
spring.Asafetyvalveisnormallysetat150psi.
Ifthepressureinthesystemrisestoapproximately
150psi,thepressurewouldforcetheballoffitsseat,
allowingthepressuretoexhaustthroughtheexhaust
portinthespringcage.Whenreservoirpressure
issufficientlyreducedtoapproximately135psi,
thespringwillforcetheballbackontoitsseat,
sealingoffthereservoirpressure.Notallsafety
valveshaveamanualreleasefeature.
Ifthesafetyvalvehastorelievepressure,thegovernor
orcompressorrequiresadjustment,serviceorrepair.
Thisshouldbedonebyaqualifiedmechanic.
Foot Valve
Thefootoperatedvalve(31)isthemeansofapplying
airtooperatethebrakes.Thedistancethetreadle
ofthefootvalveisdepressedbythedriverdetermines
theairpressurethatwillbeapplied,butthe
maximumapplicationwillnotexceedthepressure
inthereservoir.Releasingthefootvalvetreadle
releasesthebrakes.
Whenthedriverappliesthebrakes,depressingthe
treadlepartway,thefootvalvewillautomatically
maintaintheapplicationairpressurewithoutthe
driverhavingtoadjustthepressureofhisfooton
thetreadle.
Releasingthetreadleallowstheapplicationairto
bereleasedthroughtheexhaustportsintothe
atmosphere.Airtreadlesarespringloaded,producing
adifferentfeelfromhydraulicbrakeapplications.
Brake Chambers, Slack Adjustersand Brake Lining
Brake Chamber and Slack Adjuster(Brakes o)
Treadle
Treadlespring
Supply romreservoir
Exhaust Port
Push rod
Brake chamber
DiaphragmDiaphragm
return spring
Air inlet
Mounting bolts
Clevis and pin
Slackadjuster
Air pressure greater than 150 psi
Saety Valve
Foot Valve
To brakechambers
-
7/29/2019 Air Brakes Manual
19/95
Air Brake Manual
Abrakechamber(11)(14)(32)isacircularcontainer
dividedinthemiddlebyaflexiblediaphragm.
Airpressurepushingagainstthediaphragmcauses
ittomoveawayfromthepressure,forcingthepush
rodoutwardagainsttheslackadjuster.Theforce
exertedbythismotiondependsonairpressureand
diaphragmsize.Ifaleakoccursinthediaphragm,
airisallowedtoescape,reducingtheeffectiveness
ofthebrakechamber.Ifthediaphragmiscompletely
ruptured,brakesbecomeineffective.
Frontbrakechambers(32)areusuallysmallerthan
thoseintherearbecausefrontaxlescarrylessweight.
Abrakechamberisusuallymountedontheaxle,
nearthewheelthatistobeequippedforbraking.
Airpressureisfedthroughaninletport.Theair
pushesagainstthediaphragmandthepushrod.
Thepushrodisconnectedbyaclevisandpinto
acrankarmtypelevercalledaslackadjuster.
Thisconvertsthepushingmotionofthepushrod
fromthebrakechambertoatwistingmotionofthe
brakecamshaftandScams.Whentheairisexhauste
thereturnspringinthebrakechamberreturnsthe
diaphragmandpushrodtothereleasedposition.
Asindicatedbyitsname,theslackadjusteradjusts
theslackorfreeplayinthelinkagebetweenthe
pushrodandthebrakeshoes.Thisslackoccurs
asthebrakeliningswear.Iftheslackadjusters
arenotadjustedwithinthelimitations,effective
brakingisreducedandbrakelagtimeisincreased.
Iftoomuchslackdevelops,thediaphragmwilleventuallybottominthebrakechamber,and
thebrakeswillnotbeeffective.
Illustratedbelowaretwocommontypesofmanual
slackadjusters,showingthewormadjustinggear.
Push rod
Brake chamber
Diaphragm Diaphragmreturn spring
Air inlet
Mounting bolts
Clevis and pin
Slackadjuster
Manual Slack Adjusters
Ball Indent Slack Adjuster Positive Lock Slack Adjuster
Lock screw
Adjusting boltWorm shat
Worm gear
Locking collar
Spline Spline
Grease itting
Adjusting bol
Brake Chamber and Slack Adjuster (Brakes on)
-
7/29/2019 Air Brakes Manual
20/95
18 Air Brake Manual
Onmanualslackadjusters,theadjustingwormbolt
isturneduntilthebrakeliningstouchthedrumsandthenbackedoff,normallyNtoMaturn.Alocking
device,whichmaybeaspringloadedcollaroverthe
headoftheadjustingbolt,mustbedepressedwhen
thewrenchisslippedoverthebolthead.Thisis
knownasapositivelockslackadjuster.
Ortheycoulduseaspringloadedinternalcheckballto
locktheadjustment,anditmustberemovedtomake
anyadjustment.Thisisknownasaballindentslack
adjuster.Themoreoftenthedrivercheckstheslack,
thelesstheprobabilityofbrakefailure.Vehiclesrarely
losetheirbrakesbecauseofairloss;itisusually
becausetheyareoutofadjustment.
Itisthedriversresponsibilitytoensurethatbrakesare
adjustedcorrectly.Asimpleservicebrakeapplication
atlowspeedtocheckbrakeadjustmentisnotadequate.
Brakingathighwayspeedcausesbrakedrumexpansion
duetoheat,whichinturnrequiresgreaterpushrod
traveltomaintainthesamebrakingforce.Ifabrake
isoutofadjustmenttherewouldnotbeenoughreserve
strokeofthepushrodtraveltocompensatefordrum
expansion.Thiswouldcauseabrakefadeandwould
greatlyextendstoppingdistance.Iftravellingdown
ahill,thiscouldcausecompletebrakeloss.
Note:Detailedbrakeadjustmentproceduresare
outlinedinSection8.
Pushrod
Airinlet
Slackadjuster
Brake Chamber and Slack Adjuster (Brakes on)
Thrust washer
Clevis
Clevis pin (large)Clevis pin (small)
Actuator rod
Hairpin clip
Boot and strap
Actuator (adjusting sleeve)
Roller (pin)
Actuator piston
Pressure relie capscrew (pull pawl)
Pawl spring
Adjusting pawl
Worm
Worm seal
Adjusting bolt
Grease groove
Grease itting
Housing
Worm gear
Automatic Slack Adjuster
-
7/29/2019 Air Brakes Manual
21/95
Air Brake Manual
Somesystemshaveautomaticslackadjustersthat
adjustautomaticallytocompensateforbrakelining
wear,usuallymaintainingthecorrectclearance
betweenthebrakelininganddrum.Automaticslack
adjustersmustbecheckedregularlytoensurethat
correctadjustmentisbeingmaintained.Thereare
variousmakesandmodelsofautomaticslackadjusters
inuse.Primarily,theyareeitherstrokesensingor
clearancesensing.Astrokesensingadjusterwilladjust
theslackwhenitsensesthesetstrokeisexceeded.
Aclearancesensingadjusterwilladjustwhenthe
properclearancebetweenthebrakedrumandbrake
shoeisnotmaintained.Someautomaticslackadjusters
havetheabilitytobackofforincreasetheslackwhen
ithasoveradjustedthebrake.Ifavehicle
isequippedwithautomaticslackadjusters,itshould
notbetakenforgrantedthatthebrakeswillalways
beinadjustment.Thesystemisnotfoolproof.
Anumberoffactorscouldresultintheautomatic
slackadjusternotmaintainingproperslack.Therecouldbeimproperinstallation,inadequate
maintenance,deformedbrackets,worncambushings,
bentpushrods.Evenpoorvisualinspectioncanresult
inproblemsunrelatedtoadjusterfunction.When
conductingapretriporposttripairbrakeinspection,
lookforwornordamagedcomponents.Automaticslack
adjusterscanmalfunctionandnotkeepthebrakein
adjustment,especiallywhenithasbeeninservice
foralongperiodoftime.Thetwomostcommon
problemsareexcessiveprematurewearandinternal
contamination.Asanautomaticslackadjusterages
inservice,thecomponentswearthatsensewhenan
adjustmentisrequired.Theresultismorestrokeisrequiredfortheliningtocontactthebrakedrum,and
ifnotcheckedthebrakecouldbeoutofadjustment.
Ifevenasmallamountofwaterissuckedintoan
automaticslackadjustermechanismitcancause
corrosionor,inwinter,itcanfreezetheinternal
sensingcomponentsandinhibitorpreventadjustmen
Also,undercertainconditions,anautomaticslack
adjusterthatdoesnothavetheabilitytobackoff
orincreaseslack,mayoveradjustabrakecausing
ittodrag.Forexamplethiscouldtakeplacewhena
tractortrailerisnegotiatingalong,curvingdowngrad
Thedrivershouldsnubthebrakes,whichis
repeatedlyapplyingthebrakesmoderatelytomaintain
safecontrolofthevehicle.Howeveritwouldnottake
longinthisseverebrakingconditionforoneormore
ofthebrakedrumstooverheatandexpand.Theover
heatingwillphysicallyincreasethebrakedrums
diameter,andinextremeandprolongedconditions
willleadtolongerpushrodstrokestoachievethe
brakingforcerequired.Theautomaticslackadjuster
interpretsthisasaneedforadjustmentandwill
takeupslack.Whenthebrakedrumcoolsdownand
returnstonormalsizethebrakesareoveradjusted
anddragging.Atthattimethedrivershouldstopandcheckthebrakesforadjustment.Anumber
offullbrakeapplicationsabove90psiperdayare
requiredtokeeptheautomaticslackadjustersin
adjustment(seeSection8formoreinformation).
Becauseautomaticslackadjustersarenotfoolproof,
itisimportanttheoperatorofavehicleequipped
withautomaticslackadjustersbeabletomanually
adjustthem.Forinformationonmanuallyadjusting
theautomaticslackadjustersonyourvehicleconsult
themanufacturer.
Illustratedisacommontypeofbrakeassemblyused
ontruckrearaxlesandtraileraxles.Afrontaxleassemblyhasthebrakechamberandslackadjuster
mountedonthebackingplatebecauseofthe
steeringaction.
Brake drum
Brake chamber
Push rod, clevis and pin
Slack adjuster
Scam
Brake lining
Brake Assembly
-
7/29/2019 Air Brakes Manual
22/95
20 Air Brake Manual
Brakeliningmaterialisattachedtotheshoes.The
materialuseddependsonthebrakingrequirementsof
thevehicle.Brakeliningmustgiveuniformoutputof
brakeeffortwithminimumfadeathightemperatures.
Fadingorreductioninbrakingeffortoccurswhenthe
heateddrumsexpandawayfromthebrakelinings.
Thebrakeliningsalsolosetheireffectivenesswithoverheating.
ThetwistingactionofthebrakecamshaftandScam
forcesthebrakeshoesandliningsagainstthedrums.
Thebrakeliningsgenerateheatfromfrictionwith
thebrakedrumsurface.
Thethicknessofthedrumsdeterminestheamount
ofheattheyareabletoabsorbanddissipateinto
theatmosphere.Drumswornthinwillbuildup
heattooquickly.Dangerouslyundependablebrake
performancewillresultfromdistorteddrums,
weakreturnsprings,improperlining,poor
adjustment,orgreaseordirtonthelining.Drumsmustneverbemachinedorwornbeyond
themanufacturersspecification.
Wedge Brakes
Thisisanotherexampleofabrakeassemblyusedon
someairbrakeequippedvehicles.Theactionofthe
brakechamberpushrodforcesawedgeshapedpush
rodbetweenthebrakeshoerollers.Thisforcesthe
brakeshoeliningagainstthebrakedrum.
Thevehiclemaybeequippedwithasingleordual
chambersoneachwheel,dependingonthevehicles
sizeandstyle.
Thesebrakesmaybeequippedwithaselfadjusting
mechanismorwithamanualstarwheeladjuster.
Thestarwheeladjustmentismadewiththevehicle
jackedup,toinsurethatthebrakeliningsdonotdrag.Manualadjustmentofwedgebrakesisusuallydone
byaqualifiedmechanic.
Brakechamber
Adjusting wheel
Brake shoeroller
Push rod
Shoe returnspring
Brake lining
Brake shoe
Wedge Brake Single Chamber
Brakechamber
Single chamber Dual chamber
Shoe returnsprings
Brake lining
Adjusting wheel
Brakechambers
Adjusting wheel
Wedge Brakes
-
7/29/2019 Air Brakes Manual
23/95
Air Brake Manual
Disc Brakes
Theairactivatedheavytruckdiscbrakeissimilar
inprincipletothatusedonpassengervehicles.
Airpressureactsonabrakechamberandslack
adjuster,activatingthebrakes.Insteadofthecam
orwedgeusedinconventionalheavytruckdrum
brakes,apowerscrewisused.Apowerscrewworks
likeaCclamp,sothattheliningpadsexertequal
forcetobothsidesofthediscorrotor.Sometypes
ofdiscbrakeshaveabuiltinautomaticadjuster.
Discbrakesthatrequiremanualadjustmenthave
adjustmentspecificationsthatdifferfrom
conventionalScambrakingsystems.Alwayscheck
themanufacturersspecificationsbeforeadjusting.
Discbrakeassembliesmayhaveaspringparking
brakeunitattachedtotheservicebrakechamber.
AirOverHydraulic Brake Systems
Airoverhydraulicbrakesystemsweredeveloped
formediumweightvehiclesbecause:
dieselenginesdonothaveasourcefor
vacuumboostingunlesstheyareequipped
withavacuumpump. mediumweightvehiclesdonotrequire
afullairbrakesystem.
itgivestheoptionofpullinganairbrake
equippedtrailer.
Thesesystemscombinethebestfeaturesof
anairandhydraulicbrakesystem.Theyuse
hydraulicbrakesateachwheelwiththeir
reliableselfadjustersandlimitedmaintenance.
Onthesesystemstheairisusedtoeitheractuate
thehydraulicbrakesorboostthehydraulic
brakepressureasexplainedinthefollowing.
AirActuated Hydraulic Brake System
(Air Brake Endorsement Required)
Anairactuatedsystemusuallyhasthesame
componentsofastandardairsupplysystemincluding
awarningbuzzerandlight,compressor,governor,
wetanddryreservoirs,andafootvalvethatcouldbe
asingleordualtype.Thesecomponentsarefound
usuallyinthesameplacesasonafullairbrake
system.Alsothereareoneortwoairactuated
hydraulicpressureconvertersdependingonifthe
systemisasingleoradualsystem.Thissystem
consistsofanairchamberorcylinderattachedto
ahydraulicmastercylinder.Whenthefootvalve
isdepressed,theairpressureactuatesthepushrod
fromtheairunitthatpushesagainstthemaster
cylinderpiston,producinghydraulicpressure
directedthroughtubingtothewheelcylinders
actuatingthefrontandrearaxleservicebrakes.
Disc Brake
-
7/29/2019 Air Brakes Manual
24/95
22 Air Brake Manual
Itisessentialthattheoperatorofsuchavehiclehave
knowledgeofairpressurebuilduptime,governor
loadingandunloadingpressure,warningdevice
operation,andhowtodrainairreservoirsproperly
(seeSectionNine;PreTripAirBrakeInspection).
Ifanairactuatedhydraulicbrakesystemwastolose
itsairsupply,thevehiclewouldhavenoservicebrakes.
Onlytheparkingbrakewouldbeoperatingasitismechanicalandrequiresnoairpressuretooperate.
Eachvehiclemanufacturermayhavedifferentparking
brakeapplications,eitherautomaticallywhenair
pressureisreducedinthereservoir,ormechanically
byabrakeontherearofthetransmission,orwith
therearbrakesystem.Sincehydraulicbrakesystems
actuatedbyairpressureareregardedasanairbrake
system,yourdriverslicencemusthaveanairbrake
endorsementforyoutooperatevehiclesequipped
withairactivatedhydraulicbrakes.
Astherearemanydifferentsystemsinuse,referto
theoperatorsmanual.
AirBoost Hydraulic Brake System
(Air Brake Endorsement not Required)
Anairboosthydraulicbrakesystemusesairpressure
toassistbrakeforce.Thisissimilartovacuumassisted
brakesonmostpassengervehicles.Anairboostsystem
usuallyhasthesamecomponentsofastandardair
supplysystemincludingacompressor,governor,wet
anddryreservoirs.Thesecomponentsarefoundusually
inthesameplacesasonafullairbrakesystem.
Thebrakepedallinkageoperatesahydraulicmaster
cylinderthatsendshydraulicpressuretothebooster
unit.Initially,atlowpressurethehydraulicfluid
passesthroughtheboosterandbeginstopressurize
thewheelcylindersmovingthebrakeshoesouttothe
drums.Theseboosterunitsaresimilarinoperationto
HypowerorHydrovacvacuumboostersfound
onmostlightandmediumweightvehicles,butair
pressureisusedtointensifythehydraulicpressure
generatedbythemastercylinderratherthanvacuum.
Builtintotheboosterunitisahydraulicallyoperatedaircontrolvalve.
Air linesReservoirsCompressor
Foot valve
Hydraulic lines
Air brake chamber
Hydraulicwheel
cylinders
Hydraulicwheel
cylinders
Air lines
Air brakechamber
Hydraulic mastercylinder
Hydraulic mastercylinder
AirActuated Hydraulic Brake System
-
7/29/2019 Air Brakes Manual
25/95
Air Brake Manual
Thisiswhereairfromthereservoirisdirected.
Asthepressurefromthemastercylinderincreases,
theaircontrolsectionintheboosterwillopen
andbegintodeliverairpressuretotherearofthe
aircylinder.Theaircylinderpushrodtransfers
pressureonapistoninthehydraulicsectionofthe
booster,increasingthehydraulicpressureatthe
wheelcylinders.
Thedriverhasfullcontrolofthebrakingforceas
theaircontrolsectionmodulatestheboostpressure
inproportiontothemastercylinderpressure.
Ifthevehiclewastolosealloftheairpressure
thebrakesystemwouldlosetheairassistboost,
howeverthehydraulicsystemwouldcontinueto
workbutatreducedeffectiveness.Anairbrake
endorsementonadriverslicenceisnotrequired
tooperateavehiclewiththisbrakesystem.Consult
theoperatorsmanualforthevehicleyoudrivefor
maintenancerequirements.
Air lines
Hydraulicwheel
cylinders
Compressor
Reservoir Hydraulic mastercylinder
Brakepedal
Hydraulic lineBooster unit
AirBoost Hydraulic Brake System
Air lines Booster unit
Hydraulicline
Hydrauwheel
cylinde
-
7/29/2019 Air Brakes Manual
26/95
24 Air Brake Manual
Section Summary Questions
1. Whatarethefivebasiccomponentsofan
airbrakesystem?
2. Atwhatpressureshouldthegovernorcausethe
compressortoreturntoitsloadingstage?
3. Atwhatpressurewillthegovernorplacethecompressorintheunloadingstage?
4. Howisapluggedairfilterlikelytoaffectthe
aircompressor?
5. Whatcausesmoisturetoformintheair
brakesystem?
6. Whenisthecompressorabletoaccomplish
mostofitscooling?
7. Howaremostcompressorslubricated?
8. Howoftenshouldthereservoirsbedrained?
9. Isitnecessarytoallowallthepressuretoescape
fromthereservoirinordertoremovethemoisture
andsludgewhichmayhaveaccumulated?
10. Whatisthemaximumpressureavailablefor
afullbrakeapplicationatanygiventime?
11. Whatwillresultifthebrakedrumsareworn
thinorturnedtoofar?
12. Ifthegovernorvalvefailedtounloadthe
compressor,whatwouldprotectthereservoirs
frombecomingoverpressurizedandbursting?
13. Whatisthepurposeofhavingmorethan
onereservoir?
14. Whataretwofunctionsoftheslackadjusters?
15. Doestheamountofslackinthebrakelinkages
haveanyeffectonthebrakingefficiencyof
thevehicle?
16. Whatistheadvantageofkeepingthebrake
chamberpushrodtraveladjustedwithin
limitations?
17. Whatisthemostcommoncauseoflossof
effectivebrakinginanairbrakesystem?
18. DoautomaticslackadjustersonScambrakes
requirechecking?
19. Cantheadjustmentonairoperateddisc
brakesdifferfromScambrakes?
20. Whatoccurswhendrumbrakes
becomeoverheated?
21. Whatcausesbrakefade?
22. Whatisthemainfunctionofthefootvalve?
23. Whydoesthefeelofanairoperatedfoot
valvedifferfromahydraulicbrakepedal?
24. Onwhatprincipledoesadiscbrakeoperate?
25. Whattypeofairoverhydraulicbrakesystem
requirestheoperatortoholdanairbrake
endorsement?
-
7/29/2019 Air Brakes Manual
27/95
Air Brake Manual
HowtheBasicSystemWorks
Section3
-
7/29/2019 Air Brakes Manual
28/95
26 Air Brake Manual
Basic Air Brake System
Airispumpedbythecompressor(1)tothewetreservoir
(5),whichisprotectedfromoverpressurizationbya
safetyvalve(4).Thegovernor(2)controlsthepressure
inthereservoirtothebottomofthefootvalve(31).
Thedriverpushesthefootvalvetreadledownandair
pressureflowstothefrontandrearbrakechambers
(32&11).Thebrakechamberpushrodsmovetheslack
adjusters.TheslackadjustersrotatetheScams,forcing
thebrakeshoesagainstthedrums.Thiscausesfriction
thatstopsthewheels.Thedriverreleasesthefootvalve
treadleandtheairinthebrakechambersisallowedto
exhaustthroughthefootvalve,releasingthebrakes.
Thefollowingexplainstheadditionalcomponents
ofabasicairbrakesystem.Othervalveswhichare
necessarytoensuresmoothandefficientoperations
arenotincludedinthissimpledrawing.Theywill
bediscussedlaterinthemanual.
Note:Anairdryer(3)hasbeenaddedtoreduce
theamountofmoistureinthesystem.
OneWay Check Valve
Inthediagrambelow,tworeservoirsareshown(5)(10).
Topreventairfromflowingbackwardsinthesystem
towardthecompressor,aonewaycheckvalve(7)
isinstalledbetweenthereservoirs.Thisvalveallows
theairtoflowinonedirectiononly.Thevalveis
springloaded.
Pressureattheinletsideovercomesthespring
pressureandliftsthecheckvalveball,ordisc,off
itsseat.Airpassesthroughthevalvetotheoutlet.
Whenpressureattheoutletbecomesgreaterthan
attheinlettogetherwiththespringpressurethe
checkdeviceseats,preventingairfromflowingback
throughthevalve.
1
2
3 4 5 7 9 1032 11
32 31 11
Basic Air Brake System
Ball
Spring
Body
Cap nut
OneWay Check Valve
-
7/29/2019 Air Brakes Manual
29/95
Air Brake Manual
Air Pressure Gauge
Vehicleswithanairbrakesystemareequipped
withareservoirairpressuregauge(29).Thisgauge
ismountedinthecab,usuallyonthedashboardand
indicatestheairpressureintheprimaryandsecondary
ordryreservoirs.Thesupplyorwetreservoirdoesnot
usuallyhaveanairpressuregauge.Commonoperating
pressuresare80to135psi,dependingonthesystem.
Monitoringthegaugewillalertthedrivertoany
unusualchangesinairpressure.
Brake Application Gauge
Anadditionalgaugecanbeinstalledonthedashto
indicatetheapplicationairpressurewhenthebrakes
areapplied.Thisgaugecanbepipedtoindicatethe
pressureofeitherafootorhandapplication.(Hand
applicationwillbeexplainedlaterinthemanual.)
Low Pressure Warning Device
Allvehiclesequippedwithanairbrakesystemmust
haveadevicetowarnthedriveriftheairpressurein
thesystemdropstoadangerouslevel.Thisdevicecoul
bearedwarninglight,abuzzerorawigwag.Dueto
overuseorleaks,thelowpressureindicatorswitch(9)
willturnonaredwarninglightonthedashorcause
abuzzertosoundatorbefore60psi.Somevehicles
areequippedwithbothalightandabuzzertowarn
thedriverofalowairpressurecondition.
Wigwagsarenotfoundinmodernvehicleshaving
beenreplacedwitharedwarninglightandbuzzer.
Theymaystillbeinuseonoldervehicles.Thereare
twotypesofwigwaglowpressurewarningdevices
thatmaybeused.Bothtypeswilldropintothedriver
viewshouldthesystempressuredropto60psi.The
automaticwarningdevicewillriseoutofthedrivers
viewwhenthepressureinthesystemrisesabove60
psi.Themanualresettypemustbeplacedintheoutofviewpositionmanuallyandwillnotstayinplace
untilthepressureinthesystemgoesabove60psi.
Whicheverwarningsystemisused,buzzerlights
orwigwag,thedrivermuststopthevehiclein
asafeplaceimmediatelyandfindthecauseof
theairloss.Theairpressureremaininginthe
system(approximately60psi)isenoughforabrake
applicationifthedriveractspromptly.
Keepinmindifthevehiclehasspringparking
brakes,at60psithespringparkingbrakesare
partiallyappliedandyoucannotreleasethemuntil
airpressureincreases.Ifyoudonotmoveyourvehicletoasafeplaceyoumayfindyourselfstranded
inthetraveledportionofthehighway.
Stop Light Switch
Anydriverfollowingyourvehiclemustbewarned
whenreducingspeedorstoppingthevehicle.Thestop
lightswitch(25)isanairoperatedelectricswitchthat
turnsonthebrakelightsattherearofthevehicle
whenabrakeapplicationisbeingmade.
-
7/29/2019 Air Brakes Manual
30/95
28 Air Brake Manual
Quick Release Valve
Theapplicationofthebrakesinthebasicsystemwas
describedearlier.Inabasicsystem,whenthedriver
releasesthefootvalve,itwouldbenecessaryforthe
airunderpressureinthebrakechamberstoreturn
tothefootvalvetoreleasethebrakes.Thisreleasing
actionwouldbeslowedinlongwheelbasevehicles
becauseofthelongerlinesbetweenthefootvalve
andtherearbrakechambers.Aquickreleasevalve
(33)isinstalledtoallowthebrakestoreleasequickly
andfullybydischargingtheapplicationairnearthe
brakechambers.Toallowthebrakestoreleasequickly
andfullybydischargingtheapplicationairnearthe
brakechambers.
Relay Valve
Thefootvalveisusuallylocatedclosertothefront
wheelsthantotherearwheels.Thelongerthedistance
fromthefootvalvetotherearchambers,themoretime
itwilltakebeforetherearbrakesapply.Thisisknown
asbrakelag.Tocorrectthisconditiononalongwheel
basevehicle,arelayvalve(13)isinstalledneartherear
brakechambers.Alargerdiameterpipeisconnected
betweenthemainreservoirandtherelayvalve.The
airlinefromthefootvalvetotherelayvalvenow
becomesacontrolline.(Theairinthecontrolline
deadendsattherelayvalve.)Whenthefootvalveis
depressed,theairpressureinthecontrollineacts
onthetopsectionoftherelayvalve,relayingreservoir
airdirectlytotherearbrakechambersthroughthe
largerdiameterpipe.Thepressureofthereservoirair
deliveredinthiswaywillbethesameasthecontrol
pressuredeliveredbythefootvalve.
Releasingthefootvalveexhauststhecontrolairtothe
relayvalve,allowingittocutofftheflowofreservoir
airtotherearchambers.Thisinturnexhauststheair
inthebrakechambersbythequickreleasefeatureof
therelayvalve.
Manual Front Brake Limiting Valve
Forbettersteeringcontrolwhenbrakingonaslippery
roadsurface,itcanbeanadvantagetoreducethe
brakingefforttothefrontwheels.Thiscanbe
accomplishedbyinstallingacontrolvalve(35)in
thecab,andafrontbrakelimitingvalve(36)on
thefrontaxle.
Thecontrolvalveissetinthenormalpositionfor
dryroadsurfacesandthefrontbrakingapplication
airpressureisnormal.Onaslipperyroadsurface,
thecontrolvalve(35)issettotheslipperyroadposition.Inthisposition,thecontrolvalvewill
causethelimitingvalve(36)tooperate.Applying
airpressuretothefrontbrakesisthenreduced
to50percentoftheapplicationairpressure
beingdeliveredtotherearbrakechambers.
Delivery ports not shown
Manual Front BrakeLimiting Valve
Dash MountedControl Valve
Service port Lever
Exhaust port Service port
To limitingvalve
Fromlimiting
valve
Relay Valve
Quick Release Valve
-
7/29/2019 Air Brakes Manual
31/95
Air Brake Manual
Somesystemsareequippedwithanautomaticlimiting
valve(34).Thisvalvewillholdoffbrakeapplication
tothefrontwheelsfrom0to10psi,dependingon
howithasbeenpreset.Betweenthepresetpressure
and40psiofbrakeapplication,thereductionis
approximately50percent.Brakeapplicationsbetween
40psiand60psiarereducedbylessthan50percent.
Brakeapplicationsmorethan60psiarenotreduced
andfullapplicationisdirectedtothefrontwheels.
1
2
3 4
5
6 7 6 8 932 11
1135
31
32
29
1336
Service port Service port
Delivery port Delivery port
Piston spring
Inletexhausvalve springLower piston
assembly
Automatic Front Brake Limiting Valve
Basic Air Brake System with Manual Front Brake Limiting Valve
-
7/29/2019 Air Brakes Manual
32/95
30 Air Brake Manual
Theairbrakesystemdiscussedpreviouslyisfor
avehiclewithasinglerearaxle.Thediagram
illustratesanairbrakesystemforavehicle
equippedwithanautomaticfrontbrakelimiting
valve(34),aquickreleasevalve(33)andatandem
setofrearaxles.Bothaxlesofthetandemsetare
equippedwithbrakes.
Arelayvalve(13)hastwouses:toprovide
aquickerapplicationofairpressuretothetandemrearaxlebrakeswhenabrakeapplicationismade,
andtoreleasethebrakesquickerwhenabrake
applicationisreleased.
Section Summary Questions
1. Howcanthedrivertellhowmuchairpressure
isinthemainreservoirs?
2. Whatmustthedriverdowhenalowpressure
warningsystemactivates?
3. Whatisthepurposeofaquickreleasevalve?
4. Whatisthepurposeofarelayvalve?5. Whatisthepurposeofusingalargerdiameter
pipebetweenthereservoirandtherelayvalve?
6. Ifthefrontbrakelimitingvalveisinthe
slipperyroadposition,andthefootvalve
isdepressedtomakeabrakeapplicationof
30psi,howmuchpressurewillbeapplied
inthefrontbrakechambers?
7. Howisthereservoirprotectedfrom
overpressurization?
8. Whatstopspressurizedairfromflowingfrom
thedryreservoirbackintothecompressor?
9. Atwhatpressureshouldthelowpressure
warningdeviceactivate?
10. Howisbrakelagtotherearwheelsminimized?
11. Whenshouldadriverusethefrontbrake
limitingvalve?
34
33 13
Tandem Rear Axles
-
7/29/2019 Air Brakes Manual
33/95
Air Brake Manual
SpringParkingBrakes
Section4
-
7/29/2019 Air Brakes Manual
34/95
Spring Parking Brake Systems
(Single circuit system only)
Theinstallationofspringparkingbrakesandtheir
pipingarrangementsintoavehicleairbrakesystem
willvarydependingonthevehiclemake.
Springparkingbrakesmaybeinstalledonanairbrakeequippedvehicleforuseasareliableparking
brakesystem.Intheservicebrakesystem,thebrakes
areappliedbyairpressureandretractedbysprings.
Inthespringparkingbrakesystem,thebrakesare
appliedbyspringpressureandretractedbyairpressure.
Thespringparkingbrakechambersareattachedtothe
servicebrakechambersandoperatethroughthesame
linkage,thereforetheeffectivenessofthespring
parkingbrakedependsontheservicebrakeadjustment.
Thespringparkingbrakecontrolvalve(operatedby
asquare,yellowbutton)locatedinthecaballowsthe
drivertoexhaustairoutofthespringparkingbrake
circuittoapplythebrakes,orpressurizethecircuittoreleasethem.Somesystemsmayhaveanadditional
valvecontrolledbyabluebuttonthatappliesonly
thetractorspringparkingbrakesandnotthetrailer
springparkingbrakes.Thesystemcanalsoactas
anemergencybrake.Lossofairfromthesystem
mayautomaticallyapplythebrakes.
Controlvalveswillvary,dependingonthe
manufacturerandtypeofpipingarrangements.
Aspringloadedvalverequiresthatthevalvebe
pushedintoreleasethespringparkingbrakes.
Thisvalvecannotbeleftinthereleasedposition
withairpressureinthesystembelowapproximately
35psi.Anytimethereservoirpressuredropsto
approximately35psi,thisvalvewillclose
automatically,placingthespringparkingbrakesinto
fullapplication.Onsomeoldervehiclestheremaybe
asingletypeofpushpullcontrolvalvethatdoesnot
haveanautomaticreleasefeature.Toapplythespring
parkingbrakes,thevalvemustbeoperatedmanually,
eventhoughthereservoirpressurehasbeendepleted.
Duringnormaloperation,airpressurecages
(compresses)thespring,holdingitreadyfor
parkingoremergencybraking.
32 Air Brake Manual
Mounting Bolts
Springparkingbrake
chamber
Service brakechamber
Clevis and pin
Push rod
Diaphragm
Dustcap
Brakes O
Parking brakespring
Slackadjuster
Diaphragmreturnspring
-
7/29/2019 Air Brakes Manual
35/95
Air Brake Manual
Onthepretripairbrakeinspection(Section9),
youmustensurethattheparkingbrakespringis
notmanuallycagedoritwillnotexpandandapply
thebrake.Thebrakechambersshouldbechecked
forcracksanddamage.Thebrakechambershould
befittedwithadustcaptoensuredebriswillnot
enterthechamber.
Duringnormalservicebrakeoperation,
theparkingbrakespringdoesnotexpand.
Airpressurekeepsthespringcaged.
However,ittakesapproximately90psi
ofairpressuretokeepthespringscaged.
Ifreservoirairpressureisallowedtodrop
below90psithespringswillstarttomove.
Atapproximately70psithebrakeswillbe
partiallyappliedmakingitdifficultto
continuedriving.Iftheairpressuredrops
below60psithebrakeswillbefullyapplied
makingitimpossibletomovethevehicle
untilairpressureisbuiltuptorecagethe
springparkingbrakesorthespringsare
manuallycaged.
Springparkingbrakes(12),addedtothebrake
chambersoftherearaxleonthesingleunitvehicle,
areillustrated.Acontrolvalve(27)ismountedinthe
cab.Asupplylineofreservoirairispipedfromthedry
reservoirtothecontrolvalve.Openingthecontrol
valveallowsreservoirairpressuretoflowtothe
springparkingbrakechambers,releasingthem.
Service Brakes AppliedBrake On
12
1227
Using a Spring Parking Brake
Mounting Bolts
Clevis and pin
Slackadjuster
Pushrod
Diaphragm returnspring
Diaphragm
Springparking
brakechambe
Dustcap
Parking brakespring
Service brakechamber
-
7/29/2019 Air Brakes Manual
36/95
34 Air Brake Manual
Closingthecontrolvalveshutsoffthesupply
ofreservoirairpressureandexhauststhe
existingpressureinthespringparkingbrake
chambers.Thismotionallowsthespringto
expand,applyingthebrakes.
Caution:Parkingbrakesshouldbeintherelease
positionbeforemakingaservicebrakeapplication.
Afullbrakeapplication,madewhentheparking
brakesareapplied,cancompoundtheforceexerted
ontheslackadjustersandlinkageandresultin
damageorbrakefailure.Compoundingisthe
combinationoftwoforces:theforceappliedbythespringbrakechamberandtheservicebrakechamber.
Springbrakesareprimarilyusedasaparkingbrake,
butintheeventoflossofairpressureinthesystem,
theycanassistinstoppingthevehicle.Howquickly
theywillstopthevehicledependsonsuchfactorsas:
theweightandspeedofthevehicle;
thesteepnessofthegrade;
thespringforceofthespringbrakesthat
havebeeninstalled;and,
theadjustmentoftheservicebrakes.
Ifthebrakeshaveoverheated,suchasduring
mountaindrivingorhardhighwaybraking,care
mustbetakenwhenparkingthevehicle.Ifthe
springparkingbrakesareappliedwhenthebrake
drumhasexpandedbecauseofextremeheating,
whenthebrakedrumstartstocoolandcontract,
thepressureexertedbythespringparkingbrakemaycausethebrakedrumtocrackorwarp.When
parkingavehiclewithoverheatedbrakes,park
onlevelground,stoptheengineandleavethe
transmissioninthelowestgearandblockthe
wheels.Donotsetthespringparkingbrakes
untilyouhaveverifiedthebrakedrumiscool
tothetouch.
Mounting Bolts
Clevis and pin
Slackadjuster
Push rod
Diaphragmreturnspring
Diaphragm
Spring parkingbrake chamber
Dustcap
Parking brake springService brake chamber
Spring Parking Brakes AppliedBrakes On
-
7/29/2019 Air Brakes Manual
37/95
Air Brake Manual
Mechanical Release (Caging)
Somespringparkingbrakescanbereleased
mechanicallybywindingthemofforcaging
them.Cagingmeansthebrakesarebeingreleased.
Thisisachievedwithaboltthatrunsthrough
thecentreofthechamberbody,whichisturned
tocompressthespring.Itmaybenecessaryto
firstremovealockplateandstudtogainaccess
totheheadofthebolt.Othertypeshaveadust
capthatmustfirstberemovedandaboltinserted.
Insomecases,aspecialwrenchisrequired.
Instructiononhowtocageisusuallyonthe
bodyoftheparkingbrakechamber.Ifallairislost
andthevehiclehastobetowed,theparkingbrakes
canbereleasedbycagingthem.Alwaysblockthe
wheelswhencagingtheparkingbrakespring.
Warning
Springparkingbrakechambersshouldneverbe
disassembledwithoutfirstcompressingthespring
withacagingbolt.Thesespringsareunderextreme
pressureandcouldcauseseriouspersonalinjury
ifdisassemblyisattemptedbyanyonenotexperienced
inservicingtheseunits.Disassemblyofaspring
brakechambershouldonlybepreformedby
aqualifiedmechanicortechnician.
Section Summary Questions
1. Whatismeantbycompoundingthebrakes?
2. Whyarespringbrakesareliabletype
ofparkingbrake?
3. Howareparkingbrakesheldinthe
releasedposition?
4. Whatarethefunctionsofthecabmounted
parkingbrakecontrolvalve?
5. Willparkingbrakesapplyautomaticallyinallbrakingsystems?
6. Whatisthereasonforreleasingtheparkingbrake
beforemakingafullbrakeapplicationtest?
7. Whymustyoubecarefulparkingavehiclewith
overheatedbrakes?
8. Howcansometypesofparkingbrakesbereleased
withouttheuseofairpressure?
9. Whatisthedangerofdisassemblingaspring
parkingbrakeunit?
Mounting Bolts
Springparkingbrake
chamber
Service brakechamber
Clevis and pin
Slackadjuster
Push rod
DiaphragmDiaphragm returnspring
Caginbolt
Parking brakespring
Parking Brake Spring CagedBrakes O
-
7/29/2019 Air Brakes Manual
38/95
Air Brake Manual Air Brake Manual
TrailerSystem
Section5
-
7/29/2019 Air Brakes Manual
39/95
38 Air Brake Manual
Uptothispoint,thesystemdiscussedistheairbrake
systemofatruckortractor.Ifatrailerwascoupled
toatruckortractor,thebrakesofthetrailerwould
havetobeoperatedfromthetruckortractor.
Inthefollowingpagesthepowerunitofacombination
vehiclewillbereferredtoasatractor.
Glad Hands
Thistermreferstothecouplingdeviceusedtoconnect
thecontrol(service)andsupply(emergency)linesofthe
trailertothetractor.Thesecouplersconnecttogether
andlockinposition.Theyhaverubbergasketsthat
preventairfromescapingattheconnections.
Beforeconnectionismade,couplersshouldbeclean
andfreeofdirtandgrit.Whenconnectingtheglad
hands,startwiththetwogasketstogetherandthe
couplersata90angletoeachother.Aquicksnap
downwardswilljoinandlockthecouplers.Deadendcouplersshouldbeusedwheneverthevehicleis
drivenwithoutatrailertopreventwateranddirtfrom
enteringthecouplerandlines.
Iftheunitisnotequippedwithdeadendcouplers,
thegladhandofthecontrol(service)linecanbelocked
tothegladhandofthesupply(emergency)lineto
keepwateranddirtfromenteringthelines.
Thecleanertheairsupplyiskept,thelesschance
ofbrakeproblems.
Gladhandsandlinesshouldalsobesecuredtoprevent
thelinesfromchafingagainstvehiclecomponentsor
bouncingoffthevehicle.Thiscouldseriouslydamage
thegladhandsorlines.
Application Line
Theapplicationlineisreferredtoasacontrol(service)
line.Thislineisconnectedtothefootandhandvalve.
Whenthedriverdepressesthefootvalvetreadle
applicationairwillbedeliveredtothetractorbrake
chambersandtothetrailerbrakechambers.Whenthe
driverreleasesthefootvalvetreadle,theapplicationair
tothetrailerbrakechambersmustreturntothefoot
valvetobeexhaustedtotheatmosphere.
Thedisadvantagesofthissystemare:
ifthetrailerbrokeawayfromthetractor,thetrailerwouldnothavebrakes.
ifthecontrol(service)linepartedorruptured,
thetrailerbrakeswouldnotbeapplied,andthe
applicationairwouldbelostfromthetractor
ifthebrakeswereapplied.
iftheairpressureinthereservoirsislost,
therewouldbenowaytoapplythebrakes
ofthetractororthetrailer.
thetrailerbrakescannotbeappliedindependently
fromthetractorandthereisnowaytosetthe
trailerbrakeswhencouplingtothetractor.
theapplicationandreleaseofthetrailerbrakes
wouldbeslowerthanthoseofthetractor.
Thesedisadvantagesareovercomebytheaddition
ofthesupply(emergency)lineandvalvesdiscussed
inthefollowingpages.
Theillustrationshowsthepipingofaunitwithbrakes
applied,similartothetandemaxlesofthetractor.
Alsowithbrakesapplied,thetrailerhastandemaxles
equippedwithbrakechambers.
TheapplicationlinehasaTinsertedbetweenthefoot
valve(31)andthetractorsrelayvalve(13).Anairline
hasbeenconnectedfromthisTtothetrailerbyasetofcouplers(gladhands)(20).
Air line
Rubber seal
Air line
-
7/29/2019 Air Brakes Manual
40/95
Air Brake Manual
Thepurposeofthetrailerbrakehandvalve(30)isto
allowthedrivertocontrolindependentlytheamount
ofapplicationairpressuretobedirectedtothetrailer
brakes.Italsoprovidesamethodofapplyingthetrailer
brakeswhencouplingthetrailertothetractor.
Thevalve,alsoallowsthedrivertoapplythetrailer
brakesindependentlyofthetractor.Theamountof
applicationairpressuredelivereddependsonthe
amountthevalveisopenedbythedriver.(Itcannot
exceedthereservoirairpressure.)Somevalvesare
equippedwithselfreturninghandles.
13 20T
31
Application Line
30
31
Trailer Brake Hand Valve
-
7/29/2019 Air Brakes Manual
41/95
40 Air Brake Manual
Note: Thetrailerbrakehandvalveisnottobeusedfor
parking,asairmaybleedoffiftheengineisstopped
orthehandvalvemovestothereleasedposition.
Thedriverhasappliedthebrakesbyusingthefoot
valve(31).Applicationairisdirectedtothebrake
chambersofthetractorandtothetrailerbrakes
throughatwowaycheckvalve(26).Theshuttlehas
shiftedtothelowpressureside,closingoffanyair
TwoWay Check Valves
Thepurposeofatwowaycheckvalve(26)istodirect
airflowintoacommonlinefromeitheroftwosources.
Thisvalvewillpermitairtoflowfromthesourcethat
issupplyingthehigherapplicationpressures.The
shuttlewillshiftsothatthehigherpressurewillbe
directedtothetrailerthroughthecontrol(service)line.Thisvalveislocatedbetweenthefootoperatedvalve
andthehandoperatedvalve.
flowtowardthehandvalveside.Thehandvalve
(30)isintheclosedpositionandequalpressure
isbeingappliedtothebrakechambersofthe
tractorandthebrakechambersofthetrailer.
30 26
31
Air romhand valve
Air romoot valve
To trailer
Shuttle
Foot Valve Application
-
7/29/2019 Air Brakes Manual
42/95
Air Brake Manual
Inthisillustrationwiththefootvalve(31)released
andthehandvalve(30)opened,applicationairis
directedfromthehandvalvethroughatwowaycheck
valve(26),tothebrakechambers.Thetwowaycheck
valveinthisapplicationhasshiftedtothelowpressure
side,closingoffanyairflowtowardthefootvalveside.
Anytimeatrailerbrakeapplicationismadeusing
thehandvalve,thedrivermaydepressthefoot
valvetreadle.Ifthefootvalveapplicationisofa
higherpressurethanthatofthehandvalve,the
twowaycheckvalvewillshifttothelowerpressure
side,allowingthehigherpressuretobedirected
tothetractorandtrailerbrakes.Duringafoot
valveapplication,ifthedrivermakesahandvalve
applicationofahigherairpressure,thetwoway
checkvalvewilldirectthehigherhandvalveair
pressuretothetrailerbrakes.
Althoughthetrailerbrakesmaybeapplied
independentlybymeansofthehandvalve,the
maximumapplicationpressurecanonlybethe
sameas,orslightlylessthan,reservoirpressure.
Tractor Protection System
Atractorprotectionsystempreventstotallossofair
fromthetractorifthetrailerbreaksaway,orifthe
connectingairlinesbetweentractorandtrailer
becomeseparatedorruptured.Thetractorprotection
systemconsistsoftwovalves:thetractorprotection
valveandthetrailersupplyvalve.Othernamesfor
thetrailersupplyvalvearetrailersupplyvalve
andemergencyvalve.Therearetwotypesoftrailersupplyvalves.
Themostcommonistheautomatictrailersupply
valve.Thisisaspringloadedvalvethatisopened
manuallyandheldopenbyairsystempressure.
Onoldervehiclesyoumayfindamanualtrailer
supplyvalve,whichmaybeatoggletypeswitch
orapush/pulltypevalve.
Tounderstandthefunctionofthetrailersupply
valveandthetractorprotectionvalveinthesystem,
itisimportanttounderstandhowtheyoperate.
30 26
31
Hand Valve Application
-
7/29/2019 Air Brakes Manual
43/95
42 Air Brake Manual
Tractor Protection Valve
Atractorprotectionvalve(24)isnormallylocated
neartherearofthetractorcabandhastwoairlines
runningtoit,onefromthetrailersupplyvalve(28)
andtheotherfromthetwowaycheckvalve(26)
fedbythehandorfootvalve.
Italsohastwoairlines:onegoingtothesupply
(emergency)line(21),andonegoingtothecontrol
(service)line(22).Thetractorprotectionvalve
isspringloadedandrequiresapproximately45psiin
thesupply(emergency)linetoopenthevalve.This
allowscontrol(service)airtopassthroughthecontrol
(service)linetothetrailerwhilemakingabrake
application.Whenairlinesfromthetractorarecoupled
toatrailer,theopeningorclosingofthetrailersupply
valveopensorclosesthetractorprotectionvalve.
Disconnectingthesupply(emergency)linebetween
thetractorandtrailerwhilethetrailerischargedwill
causeanimmediatelossofpressureinthesupply
(emergency)line.Thesuddenlossofairpressurein
thelinewillcausethetrailersupplyvalvetoclose,
andwithnoairpressureinthelinethetractor
protectionvalvewillclose.Whenthetractorprotection
valueclosesitwillstoptheairflowtothecontrol
(service)line.
Intheeventofacontrol(service)lineruptureordisconnectionbetweenthetractorandtrailernoaction
orairlosswilltakeplaceuntilabrakeapplicationis
made.Serviceairwillbelostoutofthedisconnectedor
rupturedlinecausingthetractorsairpressuretodrop.
Atapproximately45psithetrailersupplyvalvewill
closecausinganemergencyapplicationofthetrailer
brakesandthetractorprotectionvalvetoclose.This
willstopthelossofserviceairatthedisconnectedline.
Thetractorprotectionvalvealsoprotectsthe
tractorsserviceairfrombeinglostduringnormal
brakeapplicationswhileoperatingthetractor
withoutatrailer.
To the supply(emergency)glad hand
To the control(service) glad
hand
Tractor Protection System (Open)(Trailer Charged)
Air romoot valve
Air romhand valve
Twowaycheck valve
(26)
Control(service)line (22)
Supply(emergency)
line (21)
Tractor protection valve (24)
Trailer supply valve (mounted in cab) (28)
Air romreservoir
-
7/29/2019 Air Brakes Manual
44/95
Air Brake Manual
Totesttheproperoperationofthetractorprotection
valve,ensurethetractorspringparkingbrakesare
appliedandthewheelsareblocked.Verifythesystem
isatfullpressureandthetrailersupplyvalveisopen
(trailercharged).Exitthecabanddisconnectthe
control(service)andensurethatthereisnoairloss
fromthecontrol(service)linegladhand.Enterthecab,
ensurefullreservoirpressurebetween115psiand135
psiandopenthetractorspringparkingbrakecontrol
valve.Shutofftheengineandmakeafullservicebrake
application.Airshouldescapefromthecontrol(service)
linetonolowerthan20psi,thetractorprotectionvalve
willcloseandtheairlosswillstop.Releasetheservice
brakeapplication.Closethetractorspringparking
brakecontrolvalveandensurethetrailersupplyvalve
isclosed,ifnot,closemanually.Starttheengineand
rebuildairpressure.Withairpressureatorabove90
psi,makeatrailerservicebrakeapplicationwithtrailer
handvalve.Noairshouldescapefromthedisconnected
control(service)line.Releasethebrakeapplication.
Exitthecabandreconnectthecontrol(service)line
gladhand.
Thisteststheproperopeningandclosingoperation
ofthespringportionofthetractorprotectionvalve.Atractorprotectionvalvewithabrokenreturnspring
willnotclosethevalveandwillcauselossofairduring
normalbrakeapplicationsifoperatingthetractor
withoutatrailer.
Trailer Supply Valve
Thetrailersupplyvalve(28)(usuallyaredoctagonal
button)ismountedinthecabofthetractor.Thedriver
opensthevalvebypushingorpullingthebutton,
dependingonthetypeused.
Openingthevalvepermitsreservoirairpressure
toflowthrough.Thisairpressureispipedtothe
tractorprotectionvalveandthentothesupply
(emergency)line.
Thevalveisspringloadedandwillbeheldintheopenpositionwhensufficientpressureis
reached.Iftheairpressuredropstoarange
between45and20psi,thevalvewillclose
automaticallybyspringpressure.Thedriver
canclosethevalvemanually,applyingthe
trailerspringparkingbrakes.
Tractor Protection System (Closed) (Trailer Not Charged)
Air rom
oot valve
Air romhand valve
Twowaycheck valve
(26)
Control(service)line (22)
Supply(emergency
line (21)
Tractor protection valve (24)
Trailer supply valve (mounted in cab) (28)
Air romreservoir
-
7/29/2019 Air Brakes Manual
45/95
44 Air Brake Manual
Automatic Trailer Supply Valve System
Thediagrambelowillustratesairbeingpipedfrom
thedryreservoirlinetothetrailersupplyvalve(28).
Thetractorprotectionvalve(24)isfedbytwolines:
onefromthetrailersupplyvalve(28)andonefrom
Inthediagramstheupperlineisthecontrol
(service)line(22)andthelowerlineisthesupply(emergency)line(21).
Illustratedisatractorequippedwithatrailersupply
valve(28)andatractorprotectionvalve(24).
thetwowaycheckvalve(26).Leadingofffromthe
tractorprotectionvalvearetwolines,eachwith
agladhandcoupler(20).Thesetwolinesarereferred
toasthecontrol(service)line(22)andthesupply
(emergency)line(21).
Thetrailerisnotcoupledandthetractorisbeing
operatedalone(bobtailing).Thedriverhasnotopenedthetrailersupplyvalve(28)andthehand
valve(30)isclosed.
28 26
24
22 21
30 28
31
26
24
20
22 21
20
-
7/29/2019 Air Brakes Manual
46/95
Air Brake Manual
Inthisillustration,thedriverhasmadeabrake
applicationwiththefootvalve(31)andapplication
airisbeingdeliveredtothetractorbrakechambers.
Thetwowaycheckvalve(26)hasshiftedtothelow
Ifthedriver,bymistake,appliedthehandvalve(30)
withthetrailerdisconnected,theapplicationairdirectedtothetractorprotectionvalvewouldalsobe
pressureside,allowingapplicationairtoreach
theclosedtractorprotectionvalve(24).
Thereisnoairlossfromthetractorthrough
thedisconnectedgladhands(20).
deadended.Again,noairlosswouldoccurifthetraile
supplyvalve(28)isintheclosedposition.
31
26
24
30 28
20
31
26
2430 28
20
-
7/29/2019 Air Brakes Manual
47/95
46 Air Brake Manual
Inthisillustration,thetrailerhasbeencoupled
tothetractor,andthecontrol(service)andsupply
(emergency)lineshavebeencoupledusingglad
hands(20).
Thetrailerunithasareservoir(16)installed.This
reservoirwillprovideavolumeofairnearthetrailer
brakechambersfornormaloremergencybraking.
Thetrailerreservoirisfittedwithadrainvalve(6),
thesameasatractorreservoir.
Arelayemergencyvalve(39)ismountedonthetrailerreservoirortothetrailerframenearthe
brakechambers.Therelayemergencyvalveserves
threemainfunctionsinthesystem:
1.Itrelaysairfromthetrailerreservoirtothe
trailerbrakechambersduringabrakeapplication.
Thispartofthevalveoperatesliketherelayvalve
previouslydiscussed.Italsoprovidesaquick
releaseofthetrailerbrakes.
2.Itdirectstrailerreservoirairpressuretothetrailer
brakes,causinganemergencyapplicationofthe
brakes.Thisactionwilloccurautomaticallyinthe
eventofrupturedorseparatedsupply(emergency)
airlinebetweenthetractorandtrailer,orlossofair
pressurefromthetractor.Arupturedcontrol(service)
linewouldnotcauseanemergencybrakeapplication.
Ifabrakeapplicationwasmadetherewouldbea
rapidlossoftractorreservoirair.Ifthebrake
applicationwasheldtheairpressurewoulddrop
lowenoughtocauseanemergencyapplication.Thedriveratanytimemayoperatethecabmounted
trailersupplyvalve(28)tocauseanemergency
applicationofthetrailerbrakes.
3.Ithasaonewaycheckvalvewhichstopsair
pressureinthetrailerreservoirfromgoingback
tothetractor.
20
16
6
39
28
Tractor and Trailer Coupled
-
7/29/2019 Air Brakes Manual
48/95
Air Brake Manual
Intheillustration,thecompressorhasraisedthe
reservoirairpressuretomaximum.Thedriverhasopenedthetrailersupplyvalve(28)to
allowthereservoirairpressuretobedirectedthrough
thetractorprotectionvalve(24)tothetrailer.Theair
pressurepassesthroughtherelayemergencyvalve(39)
Thisandthenextillustrationaretoonlyshowwhich
brakecomponentsandlinesareusedforafootvalve
andhandvalveapplication.
Applicationwiththefootvalve(31)inorange.
Applicationwiththehandvalve(30)inbrown.
Orangeandbrownareusedtoshowwhereairis
appliedanddoesnotrepresentthesourceoftheair.
Whenthefootvalveisdepressed,theapplicationair
tothereservoironthetrailer.Pressurewillbuild
upinthetrailerreservoirtothesamepressureasthereservoirsonthetractor.Thisisknownascharging
thetrailersystem.Thetrailersupplyvalveshould
onlybeopenwhenthetractorpressurehasbuiltup
toapproximately90psi,dependingonthevalve.
willbeappliedtothetractorbrakesandthetrailer
brakestogether.Aspreviouslyexplained,the
twowaycheckvalve(26)hasshifted,andapplication
airisbeingdirectedthroughthetractorprotection
valve(24)tothecontrol(service)line.Ifthefootvalve
isreleasedandthehandvalveisapplied,theshuttle
inthetwowaycheckvalvewouldshiftandapplication
airwouldapplythetrailerbrakesonly.
31
24
30
26
39
2428
Charging the Trailer System
Foot or Hand Valve Brake Application
-
7/29/2019 Air Brakes Manual
49/95
48 Air Brake Manual
Applicationairdirectedfromeitherthefootorhand
valvecausescontrolpressuretomovethroughthe
control(service)lineandactontherelayemergencyvalve(39).Thiscontrolpressurewillcausetherelay
emergencyvalvetodirectairfromthetrailerreservoir
(16)tothetrailerbrakechambers(14).Thepressure
directedbytherelayemergencyvalvetothetrailer
brakechamberswillbethesameasthepressure
directedtothetractorbrakechambers.Insuch
asystem,brakelagtimehasbeenminimized
withtheadditionofthetrailerreservoirandrelay
emergencyvalve.
Releaseofthefootorhandvalvestopstheflowof
applicationair.Therelayportionofthevalvereturns
toitsoriginalposition,stoppingtheflowofairpressure.Theexhaustingportsofthevalveexhaust
theairpressurefromthebrakechambers,releasing
thebrakes.Inthissystem,thebrakesofboththe
tractorandtrailercanbereleasedquickly.
Caution:Trailerbrakesmustnotbeusedtohold
aparkedvehiclethatisleftunattended.Lossof
pressurewillresultinlossofbrakes.Alwaysset
theparkingbrake.
14
39
14
22 21
16
14
39
14
16
30
31
Emergency Application
-
7/29/2019 Air Brakes Manual
50/95
Air Brake Manual
Ifatrailerthatisnotequippedwithspringparking
brakesweretoseparatefromthetractor,thecontrol
(service)line(22)andthesupply(emergency)line(21)
woulddetachfromthetractor.Thesuddenlossofair
pressureinthesupply(emergency)linewilltrigger
therelayemergencyvalve(39)whichcausesthetrailer
reservoir(16)todumpitsairdirectlytothetrailer
brakechambers(14).Thisplacesthetrailerbrakesinto
emergencyapplication.Thecheckvalveintherelay
Supply (Emergency) Line Rupture
Aruptureofthesupply(emergency)line(21)oran
uncouplingofthesupply(emergency)linegladhands
willtriggerthesameactionasdescribedabove.
Control (Service) Line Rupture
Ifthecontrol(service)line(22)isrupturedor
disconnected,noactionwilltakeplaceuntilabrake
applicationismade.Whenitismadebyeitherthe
footorhandvalve,theairlossfromthecontrol
(service)linewilllowertheairpressureinthetractors
reservoirsquickly,dependingontheamountofbraking
demanded.Thisairlosswilleventuallyactivatethe
tractorprotectionsystemwhichinturnwillexhaust
thetrailersupply(emergency)lineandcausethe
trailerrelayemergencyvalvetoapplythetrailerbrakes.
Itshouldbenotedthatanyproblemthatcauses
emergencyvalvewillclose,preventinganybleedback
pressurefromthetrailerreservoir.Thelossofpressure
inthetrailersupply(emergency)linewillcausethe
tractorprotectionsystemtosealoffthetractorleaving
sufficientairpressuretobrakethetractor.
Thetrailerbrakeswillremainapplieduntileitherthe
pressureinthetrailerreservoirandlinesisdrainedoff,orthesupply(emergency)lineisrepairedand
thesystemisrecharged.
aseveredropinreservoirpressureonthetractor
system,willcausethelowwarningdeviceto
signalthedriver.
Inthefollowingillustration,thecontrol(service)line(22)hasrupturedandthedriverismakingabrake
applicationwiththefootvalve(31).Thetractorwill
havebrakesappliedbutthetrailerwillhavenobrakes
Ifthebrakeapplicationisheld,theairpressureinthe
tractorsystemwilllowertoadangerouslylowlevelan
thenthetractorprotectionsystemwillplacethetraile
brakesintoanemergencyapplication.
21
39
16
14
14
-
7/29/2019 Air Brakes Manual
51/95
50 Air Brake Manual
Ruptureofthecompressordischargelinewouldresult
inlossofairpressurefromthesupply/wetreservoir(5).Whentheairpressureinthesupply/wetreservoir(5)
ofthetractorfallsbelowthewarninglevel,dueto
acompressorfailureorexcessiveleakageonthe
tractor,thewarningdeviceswillstarttooperate.
Inthediagram,theonewaycheckvalve(7)has
preventedthereservoirairpressureintheprimary/dry
reservoir(8)fromescapingbacktothesupply/wet
reservoirandtherupturedline.
Thereissufficientreservoirairpressureinthe
primary/dryreservoirforalimitednumberofbrakeapplicationstostopthevehiclebeforethespring
parkingbrakesareactivated.(Thiswilldependonhow
thespringparkingbrakesarepipedinthesystem.)
22
31
Control (Service) Line Rupture
5 7 8
Loss o Reservoir Air Pressure
-
7/29/2019 Air Brakes Manual
52/95
Air Brake Manual
Inthisillustrationthepressurehasbeenloweredto
approximately4520psiandthetractorprotection
systemhasclosedautomatically,placingthetrailer
brakesintoanemergencyposition.Also,thespring
parkingbrakessystemhashadtheairpressure
releasedactivatingthespringparkingbrakes.
Manual Trailer Sup