What is the Hydraulic Brake?

As motorized vehicles became more powerful, the need for more powerful brakes became important to the design of any automobile, motorcycle, or truck. Perhaps the most commonly used braking system that takes on this demand is the hydraulic brake, which uses a master cylinder, line, and caliper filled with liquid to actuate pistons that press brake pads against a moving rotor or drum. Hydraulic brake systems can be found on most automobiles and motorcycles on the road today, as well as on some bicycles.

There are several main components to the hydraulic brake. The master cylinder is the piece closest to the operator of the vehicle which contains a brake pedal or lever that pushes a piston rod into the cylinder. The airtight master cylinder then allows the liquid to be pushed into a hydraulic line or hose. These lines and hoses can be flexible or rigid, depending on the application, but almost all lines and hoses are designed to allow for minimum flexibility, which would allow the fluid inside the line to expand outward rather than directionally toward the caliper.

The caliper of the hydraulic brake can work in a few ways. On hydraulic brake systems that use a disk, the caliper is a metal housing that rests on either side of a rotor. The hydraulic line connects the master cylinder to the caliper, and as fluid is pushed through the line and into the caliper, a set of pistons are actuated inside the caliper. These pistons push inward toward the rotor. In between the pistons and the rotor sit the brake pads, which can be made of asbestos or other composite materials that are resistant to heat and brake fade.

Another type of caliper is used on hydraulic drum brakes. Instead of being mounted on the outside of a rotor, this caliper — called a wheel cylinder — is instead mounted inside a metal drum. The pistons push outward instead of inward to press the brake pads against the inside of the drum, rather than the outside of a disk.

The type of fluid used in the hydraulic brake system depends on its application. For most cars, trucks and motorcycles, DOT 4 or DOT 5 brake fluid is used. This type of fluid is glycol-ether based and is particularly resistant to heat. On some hydraulic brakes, mineral oils can be used instead, but the two fluids are not interchangeable on brake systems and should only be used on brakes designed for that particular fluid. Any hydraulic brake system must be periodically bled to ensure no air is in the lines, which may cause power loss.