Vacuum booster — description and testing (Audi 80 B3)

1. The vacuum booster is located between the brake pedal and the master cylinder, and makes it easier for the driver to press the brake pedal. Except for the 1994 cc model with automatic transmission, the vacuum for the block is taken from the intake manifold.

2. On the 1994 cc model with automatic transmission, the vacuum pump is mounted on the cylinder head and is controlled by a plunger from the camshaft.

3. On the 1781cc model with automatic transmission, the vacuum booster is included in the hose from the intake manifold to the vacuum servo.

4. The vacuum booster mainly consists of a diaphragm and a one-way valve. When the brake pedal is released, the vacuum is directed to both sides of the diaphragm, but when the pedal is depressed, one side is connected to the atmosphere. The resulting pressure difference makes it easier for the master cylinder pistons to press.

5. Usually, the vacuum booster is very reliable, but if the unit fails, it must be replaced. A failure of the booster does not affect the hydraulic system in any way, except that more force is required to press the pedal.

6. To test the vacuum booster, press the brake pedal several times with the engine off to dissipate the vacuum. Apply moderate pressure to the brake pedal, then start the engine. The pedal should move slightly if the booster unit is working properly.