The brake booster is operated by vacuum supplied by the engine or vacuum pump on the engine. When the brake pedal is released, vacuum is drawn to both sides of the internal diaphragm. As the pedal is depressed, one side of the diaphragm is open to the atmosphere causing assistance in reducing pedal effort. If the booster or vacuum pump fail, the braking system will still be operational except that much greater effort will be needed at the pedal.