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brake08 HYDRAULIC CONTROL.pdf

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BRAKE08 HYDRAULIC CONTROL
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90 LEXUS Technical Training 1. Describethepurposeofaproportioningvalve. 2. Describethepurposeofaloadsensingproportioningvalve. 3. DescribethefunctionofthebypassvalveinaLSPBV. 4. MeasurefrontandrearbrakepressureusingLSPVpressure gaugeset. 5. AdjustLSPBVrearbrakepressure. Section 8 HYDRAULIC CONTROL Lesson ObjectivesHydraulic Control Hydrauliccontrolvalvesregulatehydraulicpressuretotherearbrakes toensureefficientbraking.TypesofcontrolvalvesusedonToyotas: w ProportioningValve. w ProportioningandBypassValve. w DoubleProportioningValve. w LoadSensingProportioningValve. w LoadSensingProportioningandBypassValve. Controlofrearbrakepressureisnecessarybecauseinertia,created whenthebrakesareapplied,shiftsvehicleweighttowardthefront wheels.Becausetherearwheelshavelessweightduringbraking,they canlockup,causingthevehicletolosetractionandskidoutofcontrol. Thefrontofafront-enginevehicleisheavierthantherear,sowhenthe brakesareapplied,thevehicle’scenterofgravitytendstomove forwardbecauseofinertia.Thisaddstothefrontload,andtherear loaddecreasesasaresult.Withgreaterbrakingforce,thecenterof gravitymovesfurtherforwardandtherearloaddecreasesevenmore. Shifting Center of Gravity When the brakes are applied, the vehicle’s center of gravity tends to move forward reducing load at the rear wheels. Assumingthatthefrontandrearwheelsexertanidenticalbraking forceintheabovecondition,thereartires,whicharesubjecttoa smallerload,tendtolockupearly.Thiswillcausethereartirestolose tractionorskid. Whenthetiresskid,thefrictionbetweenthetiresandtheroad becomesextremelysmall,andthetireswillfailtoremaininsufficient contactwiththeroad.Unlessthevehicleismovingstraightahead,it willfishtail“,whichcanbeverydangerous. Hydraulic Control ValvesSection8 92 LEXUS Technical Training Thebrakingforceofthereartiresmustbereducedbelowthatoffront tiresinordertopreventearlylock-up.Thisisachievedbythe proportioningvalve(P.valve).Itisdesignedtoautomaticallyreduce thehydraulicpressurefortherearwheelcylindersinproportionto hydraulicpressurefromthemastercylinder. Proportioning Valve Location The braking force of the rear tires must be reduced below that of front tires in order to prevent early lock-up. Thegraphbelowshowsanidealhydraulicpressurecurveforthefront andrearwheels(actualvaluesvaryfromonevehiclemodeltoanother). Theproportioningvalveisdesignedtobringactualpressurecurvesas closetotheidealastechnicallypossible. Hydraulic Pressure Curve The P-valve is designed to bring actual pressure curves as close to the ideal as technically possible. Hydraulic Pressure CurveHydraulic Control ThespringintheProportioningvalveholdsthevalveintheopen position.Duringnormalbrakingthebrakefluidflowsthroughthe valvewithoutanyproportioningaction.However,whenheavier brakingoccurs,pressureonthewheelcylindersideofthe proportioningvalvepushesthevalveagainstspringtensionandcloses thevalve.Thisineffectreducespressuretotherearbrakes.As pressureincreasesonthemastercylinderside,itliftsthevalve, increasingpressuretothewheelcylindersideofthevalve.Aspressure increasesonthewheelcylindersideofthevalve,itseatsagain.This occursinrapidsuccessionaslongaspressurefromthemastercylinder increases. Proportioning Valve Pressure on the rear cylinder side pushes the valve against spring tension and closes the valve. Proportioning Valve OperationSection8 94 LEXUS Technical Training Inordertoreleasethepressurebetweentheproportioningvalveand therearwheelcylinders,thevalveseatfloatsasshowninthe illustration.Whenthepressurefromthemastercylinderisreleased, thepressuredifferenceonthevalveseatcausesittobepushedaway fromitsseatedpositiononthevalvebody.Thisallowsfluidtopassthe valveseatandthebrakestorelease. Proportioning Valve Brake Released When the pressure from the master cylinder is released, the valve seat pushed away from its seated position allows fluid to pass.Hydraulic Control Theproportioningfunctionofthisvalveisthesameasthatdescribed onthepreviouspageshowever,a BypassValve isincorporatedinto thevalvebody.Itensuresmaximumbrakingpressuretotherear brakeswhenthereisalossofbrakepressureinthefrontbrakecircuit. Thehydrauliccircuitfromthemastercylindertothefrontbrakesflows throughpartoftheproportioningvalvehousingwheretheBypass Valve monitorsfrontbrakepressure.Thespringpushesthebypassvalveto theleftandpushestheproportioningvalvetotheright,providingthe properspringtensionforproportioningvalveoperation. Rearbrakehydraulicpressurepushesthebypassvalvetotherightwhile frontbrakepressurepushesthevalvetotheleft.Theoverallhydraulic effectonthevalveisneutralandthespringholdsittotheleft. Bypass Valve Operation Spring loaded to the left, the bypass valve establishes the spring position for normal proportioning operation. Proportioning and Bypass Valve OperationSection8 96 LEXUS Technical Training Shouldthehydrauliccircuittothefrontbrakesfail,rearbrake pressurewillmovethebypassvalvetotheright,forcingthe proportioningvalvetotheright,whichallowsunregulatedpressureto applytherearbrakes. Bypass Valve Operation The bypass valve moves right when front brake pressure drops, increasing spring tension of the proportioning valve thereby ensuring maximum pressure. ThediagonalsplitbrakesystemincorporatedonallFWDvehiclesuses adoubleproportioningvalveinwhichtwovalvesarearrangedparallel tooneanotherinthesamevalvehousing.Onevalvecontrolspressure totherightrearbrakesandtheothervalvecontrolspressuretotheleft rearbrakes. Proportioning Valve on Diagonal Split Brake System All FWD vehicles use a double proportioning valve to control one front brake and one rear brake on the opposite side. Double Proportioning ValveHydraulic Control Themovementofbothvalvesiscontrolledbythetensionofonespring. Withasinglespring,abalancedpressureisappliedtoeachvalve throughtheretainer. Double Proportioning Valve Both valves are controlled by the tension of one spring. Therealadvantagetoonespringisseenwhenonehydrauliccircuit losespressure.Inthiscaseonlyonevalvecounteractsthespring tensionwhichrequiresadditionalhydraulicpressuretocompressit. Thisresultsinhigherpressuretotherearbrake,providingagreater degreeofvehiclecontrol. Rear Wheel Cylinder Pressure When one circuit fails, pressure to the rear cylinder increases higher in proportion to master cylinder pressure because only one piston compresses the spring. Pressure Loss in One CircuitSection8 98 LEXUS Technical Training TheLSPVisusedonToyotamodelssuchasTruck,VanandStation Wagonswhichmaybeusedtocarryavarietyofloads.Theheavierthe load,thegreatertheportionofbrakingisrequiredoftherearbrakes. TheLSPVallowshigherpressuretotherearbrakestoaccomplishthis. TheLSPVisattachedtothebodyorframeabovetheleftrearcontrol armoraxlehousing.Loadsensingisaccomplishedbysuspendingthe sensingspringbetweenthevehiclebodyandtherearaxlehousing.The loadsensingspringmovementcausedbyvehicleheightchangesdueto load,istransmittedtotheproportioningvalve. Load Sensing Proportioning Valve Load sensing is accomplished by suspending the sensing spring between the vehicle body and the rear axle housing. Load Sensing Proportioning ValveHydraulic Control Asavehicleisloaded,theleafspringsarecompressedasthevehicle bodylowers.Theloadsensingspringprovidesavariableforcepushing theproportioningpistonupasthevehicleisloaded.Asthepistonis lifted,ahigherbrakehydraulicpressureisrequiredtoforcethepiston downresultinginhigherpressureattherearwheels. LSPV Lever Balance The load sensing spring provides additional pressure to the proportioning valve based on vehicle load. Rearwheelcylinderpressureisadjustedaccordingtoincreasesor decreasesinvehicleload.Thepressurechangeforonerearwheelis shownbelow. LSPV Rear Wheel Cylinder Pressure Pressure to the rear brakes is regulated at a higher pressure when the vehicle is loaded. LSPV OperationSection8 100 LEXUS Technical Training Whenunloaded, a vehiclebodyrisestonormalvehicleheightandno forcefromtheloadsensingspringisappliedtothepiston.Therear wheelcylinderisregulatedatalowerpressureasshownbythelineO -A-Binthechartonthepreviouspage. Whenthefluidpressurefromthemastercylinderislowthepistonis pushedupwardbytheforceofthepistonspring.Fluidpressureis transmittedfromchamberAthroughthepassageintochamberBand totherearwheelcylinder. Whenthemastercylinderpressurerisesandpressureonthevalvetop (A2)becomesgreaterthanpistonspringtension,thepistonispushed downwardandthevalveisclosed.Hydraulicpressuretotherear cylindersatthistimewillbeasindicatedbythepointofdeflectionA“ inthegraph.Theupwardforceofthepistonspringisequaltothe downwardforceofhydraulicpressurewhenthevalveisintheclosed position. Asthemastercylinderfluidpressureincreaseswithfurtherbrake application,thepistonispushedupwardagainandthevalveopens. Pressuretotherearwheelcylindersincreaseswhenthevalveopens, butthepistonispusheddownbeforewheelcylinderpressurebecomes equaltomastercylinderpressure,andthevalvecloses. Proportioning Valve in Unloaded Position The proportioning valve is normally open. When the master cylinder pressure rises and pressure on the valve top (A2) becomes greater than piston spring tension, the piston is pushed downward and the valve is closed. Unloaded VehicleHydraulic Control Astheloadinthevehicleisincreased,thevehiclebodymovesdown,and theLSPVpistonispushedupbythelevercausingtherearwheelcylinder toberegulatedatahigherpressureasshowninthegraph(O-C-D). Whenthefluidpressurefromthemastercylinderislow,thehydraulic pressuregoingtotherearwheelcylinderisnotcontrolled.Asmaster cylinderpressurerisesandbecomesgreaterthanthecombinedspring tension,thepistonispusheddownwardandthevalveisclosed regulatingpressuretotherearbrakecylinder. Proportioning Valve in the Loaded Position The LSPV piston is pushed up by the lever causing the rear wheel cylinder pressure to increase. Loaded VehicleSection8 102 LEXUS Technical Training TheLSPBVisusedonPrevia’s,Trucks,Tacoma’s,T-100’s,Caband Chassis2WDand4WDmodels.TheLSPBVisaLSPVtowhicha bypasscircuithasbeenadded.Theoperationofthebypassvalveis similartotheProportioningandBypassValve. Whenthefrontbrakecircuitisoperatingnormally,theLSPBVvaries thepressuretransmittedfromthemastercylindertotherearwheels basedonvehicleload,inthesamewayastheLSPV.However,ifthe frontbrakecircuitfails,hydraulicpressureistransmitteddirectlyto therearwheelcylinders,bypassingtheproportioningpartofthevalve sothatenoughbrakingforcecanbeapplied. Thehydraulicsensingcircuitwhichlinksthefrontbrakehydraulic circuittotheLSPBV,ispartofthefronthydrauliccircuit.When bleedingthefrontbrakesystem,besuretobleedairfromtheLSPBV aswell,orthepedalmayfeelspongywithdiminishedbrake performance. Load Sensing Proportioning and Bypass Valve Load Sensing Proportioning and Bypass ValveHydraulic Control Whenthefrontbrakecircuitisoperatingnormally,pressurefromthe mastercylinderfrontandmastercylinderrearareequal.Thebypass pistonispushedandhelddownbythespring. Ifpressurefromthefrontbrakesfallstozero,adifferencewillexist betweenthehydraulicpressurepushingthebypassvalveupandthe pressurepushingthevalvedown.Thiscausesthebypassvalvetobe pushedupward,pushingthepistonupward,andopeningthepassage atthetopofthevalve.Thehydraulicpressurefromthemaster cylinderisnotcontrolled.Fullpressurefromthemastercylinderis transmittedtotherearwheelcylinder. Fail-safe Operation When pressure from the front brakes (Pf) is lost, Piston No. 2 rises compressing the spring and opening the valve. Bypass Valve OperationSection8 104 LEXUS Technical Training AdjustmentoftheLSPVisaccomplishedbychangingthelengthof(A) intheillustrationsbelow.Thedistancehasaninitiallengthwhichcan befoundintheRepairManual. Adjustment Length Ifdistance(A)istooshort,thehydraulicpressurebreakingpointwill decrease.Hydraulicpressuretotherearwheelcylinderswillbelower thannormal,reducingbrakingperformance. Whendistance(A)istoolong,thehydraulicpressurebreakingpoint willrise.Hydraulicpressuretotherearwheelcylinderwillbehigher thannormal,increasingthebrakingforceoftherearwheels. Toadjustthevalveproperlyandensureefficientbraking,theLSPV gauge(SST09709-29017-01)mustbeusedtomeasurethefrontand rearbrakepressure. Pressure Gauge SST To adjust the valve properly and ensure efficient braking, the LSPV gauge (SST 09709-29017-01) must be used to measure the front and rear brake pressure. LSPV AdjustmentHydraulic Control ThegaugesareprovidedintheSSTKit.Installonegaugeatthefront wheelcylinder.Theothergaugeisinstalledattherearwheelcylinder. Havingopenedthesystem,theairmustbebledfromthesystembefore accuratesystempressurescanberead.Bleedscrewsarelocatedonthe hoseendofthegauge. FollowtheprocedureoutlinedintheRepairManualtodeterminethe following: 1. Rearaxleload(basedonvehiclemodel). 2. Frontbrakepressurespecifications. 3. Rearbrakepressurespecifications. Theweightofthevehiclemeasuredattherearaxlemustbe determinedandadditionalweightaddedtomeettheRepairManual specification.Thiswillestablishtheproperrelationshipofthe proportioningvalveandtherearaxlehousing. Hydraulic Pressure Measurement Hydraulic pressure at the front brake should be compared with the pressure of the rear brake in two stages. Next,thehydraulicpressureatthefrontbrakeshouldbecompared withthepressureoftherearbrakeintwoorthreestagesasspecified intheRepairManual. w First,thefrontpressureisbroughttoaspecifiedpressure (example:1,138psi)andtherearpressureshouldbewithina specificpressurerange(example:583psito768psi). w Second,withoutreleasingthebrakepedal,t
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