Checking the brake booster (Audi A6 C5)

The brake booster reduces the force applied to the brake pedal, thereby making it easier to control the car. The principle of the brake booster is based on the use of vacuum in the intake manifold. If the brake booster fails, the performance of the brake system is not affected, but significantly more force must be applied to press the brake pedal.

Press the brake pedal several times with the engine off to create the same pressure in the vacuum booster cavities, close to atmospheric. At the same time, determine whether the valve body is jammed by the force applied to the pedal. Stop the brake pedal in the middle of its travel and start the engine. If the vacuum booster is in good working order, the brake pedal should move forward after the engine starts.

If the pedal does not move forward, remove the vacuum hose from the brake booster and start the engine. Use your finger to close the end of the hose and check if there is a vacuum in the hose.

If there is no vacuum, check the vacuum hose for leaks and replace it if necessary.

On a diesel engine, disconnect the vacuum hose from the vacuum pump and check for vacuum at the pump port. The vacuum pump on a four-cylinder engine is flanged to the rear of the cylinder block, while on a six-cylinder engine it is located on the side of the left cylinder head.

If there is no vacuum, replace the vacuum pump.