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When we step on the brake pedal, the booster's diaphragm squeezes the air in the front side of the booster. This pressure unseats the check valve that controls intake manifold vacuum. When the check valve opens, manifold vacuum sucks the air out of the booster. This "suction," called negative pressure, pulls the diaphragm forward, which applies forward pressure on the rod and master cylinder piston. The power booster helps us apply the brakes.