Press the brake pedal firmly with the engine off at least 5 times, then start the engine with the pedal depressed. The brake pedal should drop noticeably under your foot.
Otherwise, unscrew the vacuum hose from the hydraulic booster, start the engine. Place your finger on the end of the vacuum hose to check for vacuum.
If there is no vacuum: Check the tightness and integrity of the vacuum hose and, if necessary, replace the hose. Tighten all clamps.
If there is a vacuum: Measure the vacuum and, if necessary, replace the hydraulic booster (service station work).
Attention: At the same time, the non-return valve in the vacuum hose must be replaced, since if the non-return valve is defective, the membrane of the hydraulic brake booster is damaged by the penetration of fuel vapors.