Execution order
1. Press the brake pedal at least 5 times with the car stationary. Then, with the pedal pressed, start the engine. Now the pedal should be felt under your foot.
2. If not, disconnect the vacuum hose at the brake booster and start the engine. Place your finger on the end of the vacuum hose and check if there is vacuum.
3. If there is no vacuum: Check the hose for leaks. Replace it if necessary. Tighten all clamps.
4. If vacuum is present: Measure the vacuum value. Replace the booster if necessary (work of the service station).