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 to multiply the driver's pedal effort and apply that effort to the master cylinder. ^[1] It can not be used when the engine is off, mostly after a few attempts.^{[clarification needed]}