For 1990-2009 cars

The booster is a tandem vacuum-suspended unit. Without the brake pedal depressed, the booster operates with the vacuum on both sides of its diaphragms. When the brake pedal is depressed, air at the atmospheric pressure is admitted to one side of each diaphragm to provide power assist. When the brakes are released the air is shut off from that side of each diaphragm. The air is then drawn from the booster through the vacuum check valve to vacuum source.