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brake05 BRAKE BOOSTER.pdf

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BRAKE05 BRAKE BOOSTER
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1. Explainthefunctionofenginevacuuminprovidingbrakeassistto themastercylinder. 2. Performthefollowingboostertestsusingthebrakepedal: - operatingtest - airtightnesscheck - airtightnessunderload 3. UsingabrakeboosterpushrodgaugeSST,measureboosterpush rodclearanceanddetermineneededadjustment. 4. Listthesymptomsofanimproperlyadjustedboosterpushrod. Section 5 BRAKE BOOSTER Lesson ObjectivesSection 5 42 LEXUS Technical Training Thebrakeboosterisdesignedtocreateagreaterbrakingforcefroma minimumpedaleffort,usingadifferenceinatmosphericpressureand theengine’smanifoldvacuum.Itincreasesthepedalforce2to4times dependingonthesizeofthediaphragm.Thebrakeboosterislocated betweenthebrakepedalandthemastercylinder. Whenpressureisappliedtothebrakepedal,pressureisexertedonthe boosterairvalve.Withpressurecreatedbytheboosterthemaster cylinderisapplied.Shouldtheboostermalfunction,thenormal mechanicalbrakingforceofthemastercylinderismaintained. Thebrakeboosterconsistsofthe body,boosterpiston,pistonreturn spring,reactionmechanism,andcontrolvalvemechanism . Thebodyisdividedintoaconstantpressurechamberandavariable pressurechamber.Thechambersareseparatedfromeachotherbya diaphragm.Thecontrolvalvemechanismregulatesthepressureinside thevariablepressurechamber. Single Diaphragm Booster The body is divided into a constant pressure chamber and a variable pressure chamber separated from each other by a diaphragm. Brake Booster ConstructionBrake Booster Thebasicprincipleofthebrakeboosterispressuredifferential.When vacuumisappliedtobothsidesofthepiston,thepistonispushedto therightbythespringandremainsthere. Control Valve Closed When vacuum is applied to both sides of the piston, the piston is pushed to the right by the spring. WhenatmosphericairisallowedintochamberBthepistonstartsto compressthespring,duetothedifferenceinpressure,andmovesto theleft.Thiscausesthepistonrodtomovethepistonofthemaster cylinder,generatinghydraulicpressure. Control Valve Open When atmospheric air is allowed into chamber (A), the piston starts to compress the spring due to the difference in pressure. IntheOFFposition,theAirValve(connectedtotheValveOperating Rod)ispulledtotherightbytheAirValveReturnSpring.TheControl ValveispushedtotheleftbytheControlValveSpring.Thiscausesthe AirValvetocontacttheControlValve.Therefore,theatmosphericair thatpassesthroughtheaircleanerelementispreventedfromentering theVariablePressureChamber. Basic Booster Operation Booster Air Valve OperationSection 5 44 LEXUS Technical Training Thepiston’sVacuumValveisseparatedfromtheControlValveinthis position,providinganopeningbetweenpassageAandpassageB.Since thereisalwaysvacuumintheConstantPressureChamber,the openingallowsvacuumintotheVariablePressureChamber.Asa result,thepistonispushedtotherightbythepistonreturnspring. Booster Air Valve Brakes Not Applied The Vacuum Valve is open allowing vacuum on both sides of the booster piston. IntheONposition,whenthebrakepedalisdepressed,theValve OperatingRodpushestheAirValvetotheleft.TheControlValve whichispushedagainsttheAirValvebytheControlValveSpring, movestotheleftuntilittouchestheVacuumValve.Thisblocksoffthe openingbetweenpassageAandpassageB(ConstantPressure Chamber(A)andVariablePressureChamber(B)). Booster Air Valve Brakes Applied The vacuum valve is closed, cutting off the vacuum source to the variable pressure chamber.Brake Booster AstheAirValvemovesfurthertotheleft,itmovesawayfromthe ControlValve.ThisallowsatmosphericpressuretoentertheVariable PressureChamberthroughpassageB.Thepressuredifferencebetween theConstantPressureChamberandtheVariablePressureChamber causesthepistontomovetotheleft.This,inturn,causestheReaction DisctomovetheBoosterPushRodtotheleftandexertbrakingforce. Booster Air Valve Brakes Applied Air Valve opens allowing atmospheric air to enter the variable pressure chamber. Whenthebrakepedalisreleased,theValveOperatingRodandtheAir ValvearemovedtotherightbytheAirValveReturnSpringand reactionforceofthemastercylinder.ThismovementcausestheAir ValvetocontacttheControlValve,blockingatmosphericpressurefrom theVariablePressureChamber.Atthesametime,theAirValvealso retractstheControlValveSpring.TheControlValvemovesawayfrom theVacuumValve,connectingpassageAwithpassageB. ThisallowsatmosphericpressurefromtheVariablePressureChamber toflowintotheConstantPressureChamber.Thepressuredifferenceis eliminatedbetweenthetwochambersandthepistonispushedbackto therightbytheDiaphragm/PistonReturnSpring.Theboosterreturns tothereleasedposition. Released PositionSection 5 46 LEXUS Technical Training Booster Air Valve Released Position Pressure equalizes in the two chambers and the air valve is closed. Ifvacuumfailstoactonthebrakebooster,foranyreason,therewillbe nodifferenceinpressurebetweentheConstantPressureChamberand theVariablePressureChamber.WhenthebrakeisintheOFF“ position,thepistonisreturnedtotherightbythePistonReturn Spring. Whenthebrakepedalisdepressed,theValveOperatingRodadvances totheleftandpushestheAirValve,ReactionDisc,andBoosterPush Rod.Thismovementcausesthemastercylinderpistontoapplybraking forcetothebrakesystem,maintainingbrakesystemoperation. Booster Air Valve No Vacuum Although the booster loses self-energizing force when vacuum is lost, it still generates hydraulic pressure mechanically and can maintain brake system operation. Lack Of VacuumBrake Booster Thetandemtypebrakeboosterisacompactandextremelypowerful unithavingtwoConstantPressureChambersandtwoVariable PressureChambers.APistonseparateseachvariableandconstant pressurechamber.Withtwopistonsincorporatedintothisdesign,a largesurfaceareaprovidesadditionalboostwhiletakinguplessspace. WhenthebrakesarenotappliedtheAirValveandValveOperating RodarepushedtotherightbythetensionoftheAirValveReturn Spring,andstopwhentheycontacttheValveStopperKey.Sincethe AirValvepushestheControlValvebacktowardtheright,thepassage throughwhichatmosphericairfromtheaircleanerelemententersthe booster,isclosed.SincetheVacuumValveandtheControlValveare notincontactwitheachother,pressureisequalizedbetweenthetwo chambersthroughpassage(A)andpassage(B). Therefore,vacuumisappliedtoboththeConstantPressureChambers andtheVariablePressureChambers;so,thereisnodifferencein pressurebetweenthechambersonbothsidesofthepiston. Tandem Brake Booster The tandem type brake booster is a compact and extremely powerful unit having two vacuum chambers. Tandem Brake BoosterSection 5 48 LEXUS Technical Training Whenthebrakepedalisdepressed,boththeValveOperatingRodandAir Valvearepushedtothelefttogether.Asaresult,theControlValveand VacuumValvecomeintocontactwitheachother,closingpassages(A)and (B)(theconstantpressurechamberandvariablepressurechamber). Next,theAirValvemovesawayfromtheControlValve,and atmosphericairfromtheaircleanerelementpassesthroughpassage (B)andenterstheVariablePressureChamber.Thisgeneratesa pressuredifferencebetweentheVariablePressureChamberandthe ConstantPressureChamber,andthepistonsmovetotheleft. Theforcesappliedbythepistons,whichoccurduetothepressure difference,aretransmittedtotheReactionDiscviatheValveBody.They arefurthertransmittedtotheBoosterPushRod,becomingthebooster outputforce.ThecombinedsurfaceareaofpistonsNo.1andNo.2, multipliedbythepressuredifferencebetweentheConstantPressure ChamberandVariablePressureChamber,equalstheboosteroutputforce. Tandem Brake Booster - Brakes Applied The operation of the air valve and booster is the same as the single diaphragm booster. Brakes AppliedBrake Booster Thefollowingstepsaretakentodiagnosethebrakebooster. Withtheenginestopped,depressthebrakepedalnormally,several times.Thebrakepedalmustbedepressedbeforetheengineisstarted inordertoremovevacuumfromthebooster. Withthebrakepedaldepressedstarttheengine.Whentheengineis started,vacuumiscreatedandoperatesthebooster.Thiscausesthe brakepedaltogodown. Ifthebrakepedalgoesdownslightly,theboosterisoperatingnormally. Ifthebrakepedaldoesnotmove,theboosterisnotreceivingmanifold vacuum,orismalfunctioning. Booster Operating Check The brake pedal should sink when the engine starts. Booster Diagnosis Operating CheckSection 5 50 LEXUS Technical Training Starttheengineandletitrunforoneortwominutes,thenshutitoff. Nowsteponthebrakepedalseveraltimes,applyingnormalpressure. Besuretowaitaboutfivesecondsbetweeneachdepressionofthe pedal.Ifthebrakepedalreservedistanceincreaseseverytimethe pedalisdepressed,theboosterhasgoodairtightness. Booster Air Tightness Check Pedal reserve distance increases with successive pedal depressions. Thebrakepedalreservedistancechangeseverytimethepedalis depressed,becausethevacuumthatisstoredintheboosterisreduced everytimethebrakepedalisdepressed. ThebrakepedalreservedistancewillnotchangeiftheCheckValveis defective.Thecheckvalveislocatedonthevacuumboosterbodyor betweentheboosterbodyandthesourceofenginevacuum.It’spurpose istoactasaone-wayvalveandsealvacuumintheboostertoprovideat leasttwopowerassiststopsshouldtheenginestoprunning.Tocheck theCheckValveandvacuumhosepipingusethefollowingprocedure: w Removethevacuumhoseandvalvefromthebooster. w Blockthevalvewithafingerandstarttheengine. w Astrongvacuumshouldbefeltifthepipingandvalveare operating. w Thevacuummustremainunchangedforapproximatelyoneminute aftertheengineisstopped. Lackofvacuumindicatesamalfunctioninthecheckvalveorthe vacuumhosepiping.Ifthevacuumappearsnormal,theremaybea problemintheboosteritself. Air Tightness CheckBrake Booster Depressthebrakepedalwhentheengineisrunning,thenstopthe engineandwaitforabout30seconds.Ifthebrakepedalpositiondoes notchange,thebrakeboosterisfunctioningnormally.Itisdefectiveif thebrakepedalmovesup. Thebrakepedalreservedistanceremainsunchangedbecausevacuum ismaintainedintheConstantPressureChamber. Booster Air Tightness Under Load Stop the engine with the brake pedal depressed, the brake pedal should maintain the same height for more than 30 seconds. Air Tightness Test Under LoadSection 5 52 LEXUS Technical Training TheBoosterPushRodprojectsfromthefrontoftheBrakeBoosterand activatesthemastercylinder.Thepushrodisadjustableandthe clearancemustbecheckedanytimethemastercylinderorboosteris replaced.Thisisrequiredtoensurethecorrectgapbetweenthemaster cylinderpistonandtheboosterpushrod. Problemscanoccurifthepushrodisimproperlyadjusted: w Ifthegapistoosmall,itmaycausebrakedragandpremature brakewear. w Ifthegapistoolarge,itmaycausebrakedelayandreducedpedal reservedistance. Priortomakingtheadjustment: w Checkthebrakepedalfreeplaytoensuretheboosterisnot partiallyapplied. w Maketheadjustmentwiththeenginerunningtoensurethebooster hasvacuum.Theboosterbodywillchangeshapewhenavacuumis appliedandmayreducetheclearance. Booster Push Rod Gauge The push rod is adjustable and the clearance must be checked any time the master cylinder is replaced. AdjustingProcedure: 1. Placeanewgasketontheflangeofthemastercylinder.Setthe pushrodgaugeovertheendofthemastercylinderwiththe roundedendofthetoolplungertowardthepiston. 2. Pushtheplungerdownuntilitjusttouchesthebottomofthe pistonbore. 3. Turnthegaugeoverandsettheflatplungerendofthegaugeon theboosterandoverthepushrod.Thereshouldbenoclearance betweentheboosterpushrodandtheplunger. 4. Adjusttheboosterpushrodifnecessary.(Ifthebrakepedalis depressedtoexposetheadjustmentnut,besuretostarttheengine beforecheckingtheadjustedclearance.) Booster Push Rod AdjustmentBrake Booster ThepreferredmethodofadjustmentistheBoosterPushRodGauge procedurejustdescribed.Ifthespecialservicetoolisnotavailablethe measurementproceduredescribedherecanbeusedtoensurea calculatedclearancepriortoinstallationofthemastercylinder. Inthisprocedure,measurethedistancebetweenthebottomofthebore inthemastercylinderprimarypistontothetopoftheflangegasket usingadepthmicrometerorverniercaliper. 1. Measurefromtherimofthecylinderboretothenewgasketonthe flange,(measurementA“) 2. Measurefromtherimofthecylinderboretothebottomofthebore intheprimarypiston,(measurementB“) 3. SubtractAfromBwillgivethedepthofthepistonborefromthe mastercylinderflangegasket,(measurementC“) Alternate Measurement Method If the Booster Push Rod Gauge is not available, use a vernier caliper to establish proper clearance. Next,measuretheheightoftheboosterpushrod. 1. Placeaprecisionstraightedgeacrossthefaceoftheboosterbody adjacenttothepushrod. 2. Measurefromthetopofthestraightedgetothetopofthepushrod. (measurementD“) 3. Measurethewidthofthestraightedge,(measurementE“) 4. SubtractmeasurementD“fromE“willgivetheheightofthepush rod.(measurementF“) 5. ClearanceisdeterminedbysubtractingF“fromC“. 6. Adjustthepushrodtoobtainapproximately0.1mmto0.5mm clearance. Alternate Method for Booster AdjustmentSection 5 54 LEXUS Technical Training WORKSHEET 5-1 (ON-CAR) Brake Pedal Measurement Vehicle Year/Prod. Date Engine Transmission In this Worksheet you will practice the procedure for measuring pedal height, pedal free play and pedal reserve distance. Tools and Equipment: w Measuring tape. w Assortment of open-end wrenches. w Feeler gauge. w Trim removal tool. Pedal Height: 1. Pull the carpet down from the bulkhead to the foot well to reveal the asphalt melt sheet, (remove sill plate or trim as needed) 2. Using the measuring tape, measure at a right angle from the brake pedal pad to the melt sheet. 3. Record your measurement in the box below. Measured Brake Pedal Height Specification Pass/Fail 1. Is th
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