Normally used in braking systems on vehicles, a master cylinder is a tube or reservoir of fluid that supplies the pressure to a hydraulic system that eventually leads to actuation of the brakes. It is the first major component of the hydraulic system and is necessary for converting the movement from a brake pedal or lever into hydraulic pressure. In a car, the master cylinder is usually mounted in the engine compartment; on motorcycles or even bicycles equipped with hydraulic brakes, the master cylinder is mounted on the handlebar and is sometimes simply known as the brake lever.
A hydraulic braking system works by using hydraulic force to actuate pistons. This is accomplished by pushing a piston through the master cylinder, which in turn pushes hydraulic fluid through hollow brake lines. The force is then transferred to a caliper, which has pistons inside it that press the brake pads against a rotating rotor. When the force is applied, the brake pads slow or stop the rotor. While this describes only one style of brake — called a disc brake — other types of hydraulic brakes work in a similar fashion.
Cars often come equipped with disc brakes. In this case, the brake pedal acts as the piston that moves fluid in the master cylinder. Brake lines within a car's braking system are usually steel lines, though other materials that are more flexible can be used. The master cylinder on a car can be used to actuate a disc brake or a drum brake. Drum brakes work similarly to disc brakes, except instead of a caliper with pistons pushing inward against a moving disc, drum brakes use a wheel cylinder that pushes arms outward to actuate brake shoes, which in turn press against the inside of a drum. In either case, a master cylinder provides this force; on many vehicles, the same master cylinder may actuate disc brakes in the front wheels and drum brakes in the rear.
Motorcycles and bicycles use a different type of master unit that accomplishes the same thing. Since braking force on motorcycles and bicycles is applied using a hand brake, the brake lever provides the force that pushes fluid through the hydraulic system. The master cylinder is therefore mounted on the handlebar of the bike. Motorcycles and bicycles usually use a more flexible hydraulic line to transfer fluid, rather than the rigid steel lines often used on cars.