The master cylinder is a control device that converts non-hydraulic pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 (commonly from a driver's foot) into hydraulic pressure, in order to move other device(s) which are located at the other end of the hydraulic system, such as one or more slave cylinders. As piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

(s) move along the bore

Bore

Bore may refer to:* Bore , the diameter of a cylinder in a piston engine* Bore , the interior chamber of a wind instrument* Bore , a district of Ethiopia that includes the town of Bore* Boré, Mali...

 of the master cylinder, this movement is transferred through the hydraulic fluid, to result in a movement of the slave cylinder(s). The hydraulic pressure created by moving a piston (inside the bore of the master cylinder) toward the slave cylinder(s) compresses the fluid evenly, but by varying the comparative surface-area of the master cylinder and/or each slave cylinder, one will vary the amount of force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 and displacement applied to each slave cylinder (relative to the amount of force and displacement that was applied to the master cylinder).

Vehicle applications

The most common vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 uses of master cylinders are in brake

Brake

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

 and clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

 systems. In brake systems, the operated devices are cylinders inside of brake calipers and/or drum brakes; these cylinders may be called wheel cylinders or slave cylinders, and they push the brake pads towards a surface that rotates with the wheel (this surface is typically either a drum, or a disc, a.k.a. a rotor) until the stationary brake pad(s) create friction against that rotating surface (typically the rotating surface is metal or ceramic/carbon, for their ability to withstand heat and friction without wearing-down rapidly). In the clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

 system, the device which the master cylinder operates is called the slave cylinder; it moves the throw out bearing

Thrust bearing

A thrust bearing is a particular type of rotary bearing. Like other bearings they permit rotation between parts, but they are designed to support a high axial load while doing this.Thrust bearings come in several varieties....

 until the high-friction material on the transmission's clutch disengages from the engine's metal (or ceramic/carbon) flywheel

Flywheel

A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

. For hydraulic brakes or clutches alike, flexible high-pressure hoses or inflexible hard-walled metal tubing may be used; but the flexible variety of tubing is needed for at least a short length adjacent to each wheel, whenever the wheel can move relative to the car's chassis

Chassis

A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...

 (this is the case on any car with steering and other suspension movements; some drag racers and go-karts have no rear suspension, as the rear axle is welded to the chassis, and some antique cars also have no rear suspension movement).

A reservoir above each master cylinder supplies the master cylinder with enough brake fluid

Brake fluid

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure...

 to avoid air from entering the master cylinder (even the typical clutch uses brake fluid, but it may also be referred to as "clutch fluid" in a clutch application). Most modern light trucks and passenger cars have one master cylinder for the brakes which contains two piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

s; but many racing vehicles, as well as some classic and antique cars, have two separate master cylinders, each with only one piston (much like hydraulic clutches typically have only 1 piston per master cylinder). Each piston in a master cylinder operates a brake circuit, and for modern light trucks and passenger cars, usually a brake circuit leads to a brake caliper or shoe on only two of the vehicle's wheels, and the other brake circuit provides brake-pressure to power the other two brakes. For safety, this is done so that usually only two wheels lose their braking ability at the same time; it results in longer stopping distances and should be fixed immediately, but at least gives some braking ability, which is preferable to having no braking ability.