With the engine off, press the brake pedal hard at least 10 times, then start the engine with the pedal pressed. Now the brake pedal should move noticeably under your foot. This indicates that the brake booster is working properly.
Otherwise, unscrew the vacuum hose at the brake booster and start the engine. Place your finger on the end of the vacuum hose to check if a vacuum is created.
If there is no vacuum: check the vacuum hose for leaks and damage, tighten the hose if necessary. Tighten all clamps securely.
If you have a diesel engine: disconnect the vacuum hose from the vacuum pump and, placing your finger on the vacuum pump nipple, weld it until a vacuum is created.
If there is a vacuum: measure the vacuum, if necessary, tighten the brake booster (workshop work).
Caution: Always replace the check valve in the vacuum line as well, as the brake booster diaphragm may be damaged by fuel vapor penetration (if the check valve is faulty). The arrows on the check valve should point towards the intake manifold.