Checking the functioning of the hydraulic booster is carried out with a significant increase in the effort on the brake pedal to ensure the necessary braking.
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).