Vacuum servo

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A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the US it is commonly called a brake booster.

A vacuum servo, also known as a power booster or power brake unit, 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.