Press the brake pedal hard with the engine off at least 5 times, then start the engine with the pedal pressed. The brake pedal should noticeably drop under your foot.
Otherwise, unscrew the vacuum hose from the power steering, start the engine. Check for vacuum by placing your finger on the end of the vacuum hose.
If there is no vacuum: Check the vacuum hose for leaks and integrity and replace the hose if necessary. Tighten all clamps.
If there is a vacuum: Measure the vacuum and, if necessary, replace the power steering (work at a service station).
Attention: At the same time, the check valve in the vacuum hose must be replaced, since if the check valve is defective, the brake booster membrane will be damaged when fuel vapor penetrates.