Introduction:
                Power brake is the brake that magnifies a small force applied to the brake pedal into a proportionately larger force applied to slow or stop the vehicle. 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 system 

• 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.

• When the engine is running and the brake are not depressed there is vacuum on both sides the booster diaphragm (vacuum Suspended)

• As brakes are applied atmospheric pressure is allowed to enter the rear of the booster through an air control valve increasing pressure on the master cylinder.

 

Working:

• 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.

• Most automotive repair shop does not repair or rebuild brake boosters. It is 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.

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. 

• There are very few adjustments on power assisted brakes. If necessary brake pedals free travel can be checked using a ruler.
• 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.