master cylinder

noun
  1. the hydraulic pump of an automotive braking system that contains a cylinder and one or two pistons, is actuated by the brake pedal, and supplies hydraulic fluid under pressure to the brakes at each wheel.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for master cylinder

master cylinder

noun
  1. a large cylinder in a hydraulic system in which the working fluid is compressed by a piston enabling it to drive one or more slave cylindersSee also slave cylinder
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012