Power Brake Booster
WARNING: This page is about a different car, the 1994 BMW 525iT. However, it is still accessible from the selected car via links, so may be relevant.
With engine stopped, depress brake pedal 10 times. Start engine while depressing brake pedal. If brake pedal moves inward slightly, booster is functioning properly. If brake pedal does not move inward slightly, check engine vacuum, vacuum hose and "O" ring between master cylinder and booster. If okay, replace power brake booster.