Virtually all cars today use a power assisted vacuum brake system that multiplies the braking force and reduces the pedal effort. Since vacuum is always available when the engine is running, the system is simple and efficient. A vacuum diaphragm is located in front of the master cylinder and assists the driver in applying the brakes by reducing the force and movement required when depressing the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. Where the hose enters the diaphragm housing is a check valve that prevents brake release during low manifold vacuum.
Depressing the brake pedal closes the vacuum sources and allows atmospheric pressure to enter one side of the diaphragm. This causes the master cylinder pistons to move and act on the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and the springs return the diaphragm and master cylinder pistons to their original position.
If the vacuum supply fails, the brake pedal rod will come into contact with the master cylinder drive rod and the driver can apply the brake without any power booster. Note that it takes a lot more pedal force to stop the car and that the pedal feels "heavier", than usual.