Vacuum servo

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A vacuum servo (also called a brake booster, power booster, or power brake unit) is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. It uses a vacuum, usually supplied by the engine, to multiply the driver's pedal effort and apply that effort to the master cylinder.^[1]

Because the servo depends on the vacuum supplied by a running engine, a check valve is typically used in the vacuum line to maintain residual vacuum without engine support, allowing limited use even after parking.