Vacuum servo drive — description and testing (Audi 100 C3)

1. The vacuum servo is located between the brake pedal and the master cylinder and makes it easier for the driver to press the brake pedal. The unit operates from the vacuum from the intake manifold, on some models the vacuum is increased by a vacuum booster in the vacuum supply.

2. The unit basically consists of a diaphragm and a non-return valve. When the brake pedal is released, vacuum goes to both sides of the diaphragm, but when the pedal is pressed, one side opens to the atmosphere. The resulting pressure difference helps push the master cylinder pistons.

3. Usually, the vacuum servo is very reliable, but if the unit still stops working, it must be replaced. The failure does not affect the hydraulic system in any way, except that it is necessary to apply more force to the pedal.

4. To test the vacuum servo, press the brake pedal several times with the engine off to release the vacuum. Press the brake pedal moderately, start the engine. The pedal should move down slightly if the servo unit is working properly.