Power Brake Systems Power Brake Systems Most 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 .
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Power Brake Systems
Vacuum 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.
Intake 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 chamber
Brake Booster rear chamber
Vacuum supply to rear chamber
Air Valve assembly
NOTE:Vacuum is closed off to the rear chamber when brake are applied
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 diaphragm
B. 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.
Caution:Only in rare circumstances will it be necessary to adjust brake pedal free travel
2. Vacuum for the power brake is obtained from:
A. Master Cylinder
B. Power steering pump
C. Intake manifold
3. What is used to prevent loss of vacuum during wide open throttle?
A. Vent inside the master cylinder
B. One way check valve inside the brake booster
C. Throttle restrictor inside the intake manifold
D. Vent inside the power train control module.
4. To remove vacuum from the power brake booster you should:
A. Remove the master cylinder
B. Remove the battery negative cable
C. Remove the battery positive cable
D. Pump the brakes several times.
A. Brake lines
B. Brake pedal support bracket
C. Brake hold down springs
D. Brake pedal pushrod