Power Brake Assist: Description and Operation

The vacuum assist diaphragm assembly multiplies the force exerted on the master cylinder piston in order to increase the hydraulic pressure delivered to the wheel calipers or cylinders, while decreasing the effort necessary to obtain acceptable stopping performance.
Vacuum assist units get their energy by opposing engine vacuum to atmospheric pressure. A piston, cylinder and flexible diaphragm utilize this energy to provide brake assistance. The diaphragm is balanced with engine vacuum until brake pedal is depressed, allowing atmospheric pressure to unbalance the unit and apply force to the brake system.