Vacuum Booster Operation
Vacuum brake assist units use a large round flexible diaphragm enclosed in an assembly to create two distinct vacuum chambers. The pedal side of the diaphragm has two valves attached to the pedal arm. These valves work together to control booster operation.
A brake booster needs the right amount of vacuum to operate correctly. This is typically engine manifold vacuum (17-21Hg). On some engines, particularly diesels, an auxiliary pump is used to create vacuum. This vacuum is applied to both sides of the booster. Both the pedal side and the master cylinder side.
As the driver presses the brake pedal, the vacuum valve blocks the vacuum source located on the pedal side of the booster. At the same time the air valve moves allowing atmospheric pressure in. Sometimes a slight hissing noise can be heard as the air rushes past the filter, through the air valve and into the chamber.
This collapse in vacuum on the pedal side accompanied with the vacuum source still applied to the master cylinder side produces a powerful brake assist. This is because of the pressure differential caused by the closing vacuum valve and the opening air valve. The flexible diaphragm located in the center of the assembly provides an air tight seal. A rod or "power piston" is connected to this diaphragm. This push-rod is mechanically connected to the diaphragm and through the entire unit. One end is connected directly to the master cylinder and the other is connected to the brake pedal. This mechanical connection is why vacuum brake booster failure will not result in a complete loss of braking action. There is always a connection between the two.
When the driver releases the brake pedal, a return spring located in the center of the master cylinder side of the booster assembly returns the diaphragm to its resting position. While the spring moves the diaphragm, the air and vacuum valves are also returned to their normal positions. This returns vacuum to both sides of the diaphragm. This is known as it's balanced or at rest position. Brake booster failure will leave a vehicle with a hard brake pedal. Remember, vacuum assist brake units require sufficient vacuum to operate correctly. Sometimes this vacuum is blocked at the intake manifold port. A kink in the check valve hose will also result in this hard brake pedal condition. Cleaning this port or repairing the check valve's hose will restore vacuum and brake booster operation.