Start the engine. The pedal should move a few more millimeters. If not, the reason is as follows:
The vacuum hose from the intake manifold to the brake booster is leaking: in this case, be sure to replace the hose and check its connecting flanges.
The check valve in the vacuum hose is faulty: To check the valve, remove the vacuum hose from the brake booster and let the engine idle. If you do not hear a rhythmic sucking sound, place your finger on the free end of the hose. If a vacuum does not form in the hose, the valve is faulty.
Porous rubber gasket between the master cylinder and the brake booster: To replace, remove the master cylinder from the brake booster and replace the gasket.
The air filter on the hydraulic lifter of the brake booster is dirty: Use a wire hook to remove the filter from the hydraulic lifter. Cut the new filter to the center and press it into place around the hydraulic lifter. Make sure that the filter is closed around the lifter, otherwise unfiltered air can get into the brake booster.
The booster membrane is faulty: repair is not possible. "Make friends" with the new brake booster assembly.
Brake booster: 1 - expansion tank with brake fluid; 2 - brake booster; 3 - push rod; 4 - master brake cylinder.