2Power Brake SystemsMost all modern vehicles use power assisted brakes.A vacuum line from the intake manifold to the brake booster provides a source of vacuum.A brake booster check valve prevents loss of vacuum during wide open throttle.
3Power Brake SystemsVacuum in the intake manifold is applied to a rubber diaphragm on the vacuum side of the booster chamber (red speckled area)The piston rod (7) is pushed in when the brakes are applied and vacuum is supply to the brake booster assembly rear section through the bell valve.
4Power Brake SystemsWhen the engine is running and the brake are not depressed there is vacuum on both sides the booster diaphragm (vacuum Suspended)As brake are applied atmospheric pressure is allowed to enter the rear of the booster through an air control valve increasing pressure on the master cylinder.
5Intake manifold vacuum When the brake pedal is depressed the air valve moves forward allowing atmospheric pressure to enter the rear of the brake booster assembly.The higher pressure in the rear forces the diaphragm to move forward increasing the pressure applied to the master cylinder.A spring in the front chamber forces the diaphragm rearward when the brake pedal is released.Brake Booster front chamberBrake Booster rear chamberVacuum supply to rear chamberAir Valve assemblySpringNOTE: Vacuum is closed off to the rear chamber when brake are applied
6Power Brake SystemsMost automotive repair shop do not repair or rebuild brake boosters. Its usually more cost efficient to replace defective brake booster assemblies.It is important to understand how a power brake booster operates before diagnosing a defective booster assembly.
7Power Brake Systems To diagnose a power brake assembly 1. Pump the brake several times with the engine off to remove vacuum from the booster.2. Start the vehicle the brake pedal should move downward slowly as vacuum begins to build.3. A hard brake pedal can be caused by:A. Defective brake booster diaphragmB. Low vacuum to the brake booster.NOTE: In case the engine should stall while driving, Vacuum booster are design to maintain vacuum providing the operator time to stop safely.
8Power Brake SystemsThere are very few adjustment on power assisted brakes. If necessary brake pedal free travel can be checked using a ruler.Master cylinderCaution: Only in rare circumstances will it be necessary to adjust brake pedal free travelFirewallBrake pedalRuler
9Power Brake SystemsIf a push rod is adjusted to short it will cause excesses brake pedal free travelIf a push rod is adjusted to long it could cause brake to:Heat up due to draggingLock-up due to brake expanding caused by excesses heat.
10Power Brake QuizTechnician A says low vacuum will cause a harder brake pedal. Technician B says a defective booster diaphragm will cause a hard brake pedal. Who is correct?Technician A onlyTechnician B onlyBoth A and BNeither A nor B
11Power Brake Quiz 2. Vacuum for the power brake is obtained from: A. Master CylinderB. Power steering pumpC. Intake manifoldD. Battery
12Power Brake Quiz3. What is used to prevent loss of vacuum during wide open throttle?A. Vent inside the master cylinderB. One way check valve inside the brake boosterC. Throttle restrictor inside the intake manifoldD. Vent inside the power train control module.
13Power Brake Quiz4. To remove vacuum from the power brake booster you should:A. Remove the master cylinderB. Remove the battery negative cableC. Remove the battery positive cableD. Pump the brakes several times.
14Power Brake Quiz5. What component connect the master cylinder to the brake pedal?A. Brake linesB. Brake pedal support bracketC. Brake hold down springsD. Brake pedal pushrod