Vacuum Brake Booster: Description and Operation

There are two different power brake booster designs, although externally they appear the same. All vehicles use a 205 mm tandem diaphragm power brake booster. The two boosters are internally tuned differently depending on whether the vehicle is equipped with the standard front disc/rear drum brake combination or the optional front disc/rear disc (four-wheel disc) brake combination. If the power brake booster requires replacement, be sure it is replaced with the correct part.







The power brake booster can be identified by the tag attached to the body of the booster assembly. This tag contains the following information: The production part number of the power brake booster, the date it was built and who manufactured it.
The power brake booster reduces the amount of force required by the driver to obtain the necessary hydraulic pressure to stop the vehicle.






The power brake booster is vacuum-operated. The vacuum is supplied from the intake manifold on the engine through the power brake booster check valve.
As the brake pedal is depressed, the power booster input rod moves forward. This opens and closes valves in the power brake booster, allowing atmospheric pressure to enter on one side of a diaphragm. Engine vacuum is always present on the other side. This difference in pressure forces the output rod of the power booster out against the primary piston of the master cylinder. As the pistons in the master cylinder move forward, hydraulic pressure is created in the brake system.
The power brake vacuum booster assembly mounts on the engine side of the dash panel. The booster input push rod connects to the brake pedal. A vacuum line connects the power booster to the intake manifold. The master cylinder is bolted to the front of the power brake booster.