1. With the engine off, press the brake pedal at least 10 times, then start the engine with the pedal pressed. Now the brake pedal should move noticeably. In this case, the booster is working properly.
2. Otherwise, disconnect the vacuum hose at the booster and start the engine. Put your finger on the end of the hose and check if there is vacuum.
3. If there is no vacuum: Check the vacuum hose for leaks and damage, replace it if necessary. Tighten all clamps.
4. Diesel Engine: Disconnect the vacuum hose from the vacuum pump and check with your finger if there is vacuum. The vacuum pump is attached to the cylinder head.
5. If there is vacuum: Measure the vacuum value, replace the brake booster if necessary (work of the service station).
In a gasoline engine, be sure to also replace the check valve in the vacuum line, since the diaphragm in the brake booster may be damaged by penetrating fuel vapors if the check valve is faulty.