Vacuum Brake Booster

A faulty brake booster results in a hard brake pedal.

Brake power assist units reduce pedal effort on automotive brake systems. When the booster fails to operate correctly, the brake pedal feels hard, and greater effort is needed to operate the brakes. This is in stark contrast to a faulty master cylinder that typically results in a brake pedal that fades to the floor. Worn master cylinder cup seals cause this internal leak.

Vacuum brake assist units require the proper amount of vacuum to operate correctly. This is intake manifold vacuum (17-21 "hg), and on some engines, particularly diesel, an auxiliary pump can be used. This vacuum is applied to both sides of the booster: the pedal side and the master cylinder side.

Brake booster applied.

As the driver presses the brake pedal, the vacuum check valve blocks the vacuum source located on the peddle side of the booster. At the same time, it moves the air valve, allowing in atmospheric pressure. 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 by the vacuum source still applied to the master cylinder side produces a powerful brake assist. This is because of the flexible diaphragm located in the center of the assembly.

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 is why vacuum brake booster failure does not result in a complete loss of braking action. There is always a mechanical connection between the two.

Brake booster at rest.

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 at-rest positions. This returns vacuum to both sides of the diaphragm. It's known as its balanced or at-rest position. Brake booster failure results in a hard brake pedal, making it much more difficult to stop. A poor or restricted vacuum source also causes a hard brake pedal, robbing the booster of its vital vacuum source. A faulty air check valve causes the same problem.