1. With the engine off, press the brake pedal at least 10 times, then start the engine with the pedal depressed. The brake pedal should now feel audible. In this case, the amplifier is OK.
2. Otherwise disconnect a vacuum hose at the amplifier and start the engine. Put your finger on the end of the hose and see if there is a vacuum.
3. If there is no vacuum: Check the vacuum hose for leaks and damage, replace if necessary. Tighten all clamps.
4. Diesel engine: Disconnect the vacuum hose from the vacuum pump and test with a finger to see if there is a vacuum. The vacuum pump is attached to the cylinder head.
5. If there is a vacuum: Measure the vacuum, if necessary, replace the brake booster (service station work).
For a petrol engine, be sure to also replace the non-return valve in the vacuum line, since the diaphragm in the brake booster can be damaged due to penetrating fuel vapors if the non-return valve is defective.