With the engine off, depress the brake pedal strongly at least 10 times, then start the engine with the pedal depressed. Now the brake pedal under the foot should be perceptibly applied. This indicates the serviceability of the brake booster.
Otherwise, unscrew the vacuum hose at the brake booster and start the engine. Applying a finger to the end of the vacuum hose, check that a vacuum is being created.
If there is no vacuum: check the vacuum hose for leaks and damage, lure the hose if necessary. Tighten all clamps securely.
In the presence of a diesel engine: disconnect the vacuum hose from the vacuum pump and put your finger on the vacuum pump fitting, boil, and a vacuum is created.
If vacuum is present: measure vacuum, lure brake booster if necessary (workshop work).
Attention: When doing this, always also replace the non-return valve in the vacuum line, as the diaphragm of the brake booster can be damaged due to the penetration of fuel vapours (with a faulty non-return valve). The arrows on the check valve must point towards the intake manifold.