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This article needs additional citations for verification. (February 2017)
A vacuum servo also known as a power booster or power brake unit uses a vacuum to multiply the drivers pedal effort and apply that effort to the master cylinder.  It can not be used when the engine is off, mostly after a few attempts.
- What is a brake booster?
- "How Power Brakes Work", (a 4-page basic tutorial article, with illustrations & animation) at HowStuffWorks.com