A power booster or power brake unit uses a vacuum to multiply the driver’s pedal effort and apply that to the master cylinder. This increases the pressures available from the master cylinder.
Units on petrol/gasoline engines use the vacuum produced in the intake manifold. Vehicles with diesel engines cannot use manifold vacuum so they are fitted with an engine-driven vacuum pump.The most common booster operates between the brake and master cylinder. It increases the force that acts on the master cylinder. Whenever the brake pedal is depressed, a pushrod opens the vacuum-control valve. Vacuum from the engine lowers the pressure in the chamber, forcing the diaphragm forward and increasing the pressure on the master cylinder pistons. The level of assistance this power-boost gives depends on the pressure applied to the brake pedal.