Another major element of the power brake is the check valve. With the help of a rubber hose the check valve is connected to the engine and acts as a one-way valve that allows vacuum to enter the booster but does not let it escape. If the engine is stopped or a leak forms in a vacuum hose, the check valve ensures that air does not enter the vacuum booster. Because of this valve the vacuum booster provides enough strength to the driver to make several stops in the event that the engine stops running.
The booster is an empty shell that is divided into two chambers by a rubber diaphragm. When the foot of the driver is off the brake pedal, the valve in the diaphragm remains open so that vacuum is allowed to fill both the chambers. When the driver depresses the brake pedal, the valve in the diaphragm closes, separating the two chambers and another valve opens to allow air in the chamber on the brake pedal side.
However, there are some other factors which contribute to a loss of power assist. The engine must be running in order to have power assist. If the engine stops or stalls while driving, there will have small reserve of power assist for two or three pedal applications but, after that, it will be very difficult to depress the brakes.