This booster is a tandem vacuum suspended unit. In a normal operating mode, with service brakes in the released position, the tandem vacuum suspended diaphragms. When the brakes are applied, air at atmospheric pressure is admitted to one side of each diaphragm to provide the power assist. When the service brake is released, the atmospheric air is shut off from the one side of each diaphragm. The air is then drawn from the booster through the vacuum check valve to the vacuum source.