The power brake booster system has a dual diaphragm. In a normal operating mode, with the service brakes in the released position, the booster has vacuum on both sides of the diaphragms. When the brakes are applied, air at atmospheric pressure is admitted to the back side of the diaphragms to provide power assist. When the service brake is released, the atmospheric air is shut off from one side of the diaphragms. The remaining air is then drawn from the booster through the vacuum check valve to the vacuum source, once again creating a vacuum on both sides of the booster's diaphragms.