Brake Master Cylinder

The master cylinder is a hydraulic control device that converts physical pressure (commonly from a driver's foot) into hydraulic pressure to operate other device(s) in the hydraulic system. The most common automotive uses of master cylinders are in brake and clutch systems. The operated device in the clutch system is called the slave cylinder. In brake systems, the operated devices are brake calipers and/or wheel cylinders.
A master cylinder is made up of a reservoir of fluid, two pistons, and two or more circuits (brake lines). When the brake pedal is depressed, the pistons move to create pressure in the cylinder. This pressure compresses the fluid evenly through each circuit and into the brake calipers. Typically, engine vacuum is used to assist in applying pressure to the system. In case of failure of a brake line, the master cylinder will redirect hydraulic pressure to the remaining lines, thereby avoiding catastrophic failure of the brakes.