ASE 5 - Brakes

Module 5Balance Control Systems

AcknowledgementsGeneral Motors, the IAGMASEP Association Board of Directors, and RaytheonProfessional Services, GM's training partner for GM's Service Technical College wish tothank all of the people who contributed to the GM ASEP/BSEP curriculum developmentproject 2002-3. This project would not have been possible without the tireless efforts ofmany people. We acknowledge:

• The IAGMASEP Association members for agreeing to tackle this large project tocreate the curriculum for the GM ASEP/BSEP schools.

• The IAGMASEP Curriculum team for leading the members to a single vision andimplementation.

• Direct contributors within Raytheon Professional Services for their support oftranslating a good idea into reality. Specifically, we thank:

– Chris Mason and Vince Williams, for their leadership, guidance, and support.– Media and Graphics department under Mary McClain and in particular, Cheryl

Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & NancyDeSantis.

– For his help on the Brakes curriculum volume, Subject Matter Expert, JohnFisher, for his wealth of knowledge.

Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEPColleges for their contribution for reformatting existing General Motors training material,adding critical technical content and the sharing of their expertise in the GM product.Separate committees worked on each of the eight curriculum areas. For the work on thisvolume, we thank the members of the Brakes committee:

– George Behrens, Monroe Community College– Lorenza Dickerson, J. Sargeant Reynolds Community College– Tim McCluskey, Dakota County Technical College– Wayne Musser, Harrisburg Area Community College– Vince Williams, Raytheon

ContentsModule 5 – Balance Control SystemsAcknowledgements .......................................................................................... 2

Balance Control Systems................................................................................................ 4Metering Valve ................................................................................................................ 4Pressure Proportioning Valve ......................................................................................... 5Height Sensing Proportioning Valve................................................................................ 7Combination Valve .......................................................................................................... 8

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-4

Student WorkbookBalance Control SystemsThe brake balance control system helps to balance braking between thefront and rear wheels. Balance controls are required to compensate forvehicle load and system design. Balance controls include:• Metering valve• Fixed pressure proportioning valve• Height sensing pressure proportioning valve• Combination valve which includes the pressure differential switch

Metering ValveThe metering valve is used on rear-wheel drive vehicles equipped withfront disc/rear drum brakes (Figure 5-1). The valve is in the hydrauliccircuit to the front disc brakes. It is located in the combination valve block(Figure 5-2).

Figure 5-1, Metering Valve

Figure 5-2, Holding MeteringValve Stem

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-5

Student WorkbookDisc brakes react more quickly to initial brake application than drumbrakes due to the tighter hardware clearances. The metering valve delaysfront brake application for a fraction of a second so rear brakes actuate atthe same time as the front brakes. The delay increases vehicle stabilityduring braking.In some instances, the metering valve can prevent fluid flow to the frontwheels during pressure bleeding. Therefore the valve may have to bedisabled when pressure bleeding. Special tools are available to disablethe valve by depressing the valve's pin.

Metering Valve OperationUnder low pressure (0-25 psi), the metering valve generally remains open.From 25 to 100 psi, the metering valve seals, closing off brake fluid fromthe front brakes. The master cylinder delivers brake pressure to the rearbrakes only. Hydraulic pressure acts against the drum brake return springsto bring the brake shoes in contact with the drums.Above 100 psi, the hydraulic pressure begins to overcome valve springpressure, opening the metering valve seal. Metering valve pressureoperation may vary dependingupon brake system design.

Important: Depressing the metering valve pin disables metering valveoperation. After bleeding brakes, always remember to remove themetering valve pin depressor tool before road testing or returning thevehicle to the customer.

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-6

Student WorkbookPressure Proportioning ValveProportioning valves improve front-to-rear brake balance during hardbraking (Figure 5-3). GM vehicles have been equipped with two types ofproportioning valves:• Fixed pressure• Height-sensingDuring a hard stop, a portion of the vehicle weight is transferred to thefront wheels. The resulting loss of weight on the rear wheels couldcontribute to premature rear wheel lockup, reducing directional stability.The proportioning valve reduces brake pressure to the rear brakes duringhard braking.

Fixed Pressure Proportioning ValveThe fixed pressure proportioning valve is located in the rear brakehydraulic circuit. Fixed pressure proportioning valves can be:• Integral with the master cylinder• Screwed into the master cylinder outlet ports• Inside the combination valve• In series with the brake pipes under the vehicle

Figure 5-3, Fixed Pressure Proportioning Valve

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-7

Student Workbook

Figure 5-4, Fixed Pressure Proportioning Valve - Most Braking

Figure 5-5, FIxed Pressure Proportioning Valve - Hard Braking

During most braking, the passage to the rear wheel circuit remains open(Figure 5-4).

Under hard braking, a sudden pressure increase closes the outlet to therear wheels (Figure 5-5).

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-8

Student WorkbookHeight Sensing Proportioning ValveThe height sensing proportioning valve is used primarily on trucks andvans (Figure 5-6). The valve is mounted on the frame near the rear axle. Itis installed in the rear brake hydraulic circuit. A heavily-loaded truck is lesslikely to lock the rear wheels during hard stops and requires more brakepressure.

The height sensing pressure valve:• Senses the load on the rear axle during braking by sensing chassis

height above the axle• Changes pressure proportioning during braking accordingly

Figure 5-6, Height Sensing Proportioning Valve Operation

© 2002 General Motors CorporationAll Rights Reserved

ASE 5- Brakes

Module 5 -Balance ControlSystems

5-9

Student WorkbookCombination ValveSome vehicles are equipped with a combination valve that contains(Figure 5-7):• A metering valve, delaying front brake application• A proportioning valve, adjusting front-to-rear brake balance• Pressure differential warning switchAlthough the pressure differential, metering, and proportioning valveswere originally developed as separatecomponents, they may be incorporated into a single combination valve.The combination valve is used primarily on vehicles having both:• Rear wheel drive• Disc and drum brakes

Figure 5-7, Combination Valve