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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

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power brake systems2
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.
power brake systems3
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.

Power Brake Systems
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Power Brake Systems
  • When 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.

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

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Power Brake Systems
  • Most 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.
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Power 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 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.

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Power Brake Systems
  • There are very few adjustment on power assisted brakes. If necessary brake pedal free travel can be checked using a ruler.

Master cylinder

Caution:Only in rare circumstances will it be necessary to adjust brake pedal free travel


Brake pedal


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Power Brake Systems
  • If a push rod is adjusted to short it will cause excesses brake pedal free travel
  • If a push rod is adjusted to long it could cause brake to:
    • Heat up due to dragging
    • Lock-up due to brake expanding caused by excesses heat.
power brake quiz
Power Brake Quiz
  • Technician 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 only
    • Technician B only
    • Both A and B
    • Neither A nor B
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Power Brake Quiz

2. Vacuum for the power brake is obtained from:

A. Master Cylinder

B. Power steering pump

C. Intake manifold

D. Battery

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Power Brake Quiz

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.

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Power Brake Quiz

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.

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Power Brake Quiz
  • 5. What component connect the master cylinder to the brake pedal?

A. Brake lines

B. Brake pedal support bracket

C. Brake hold down springs

D. Brake pedal pushrod