What is a Brake Master Cylinder?

The master cylinder converts the force a driver applies to a brake pedal into hydraulic pressure in a dual braking system. Modern cars feature a dual master cylinder which means that there is a separate section for the front and rear brakes, although both are contained in a single unit.

Master Cylinder in Modern Cars

Older cars had master cylinders with one piston to accommodate all four brakes. Current safety laws have made these obsolete, according to Puhn. These are now found only on race cars. Today's modern cars use tandem master cylinders with two pistons, and each section acts a single cylinder. According to Puhn, the piston closest to the brake pedal is the primary piston, which usually operates the front brakes, and the other is called the secondary piston, which operates the rear brakes.

Operation

According to "High-Performance Brake Systems," by James Walker, Jr., the simplest master cylinder design "uses a single piston acting against a fluid-filled cavity in a machined body." As part of a hydraulic braking system, when a driver's foot hits the brake, the piston moves forward in the master cylinder by a pushrod attached to the brake pedal. In the simplest terms, fluid is then pushed out of the master cylinder into the brake line to the caliper and against the brake pad to stop the car.

Reservoir

The master cylinder is comprised of a hydraulic fluid reservoir usually directly above the piston, according to the "Brake Handbook" by Fred Puhn. Inside the reservoir there are two holes, called the fluid-inlet port and the compensating port, that allow fluid to enter the cylinder. On older cars, a reservoir diaphragm covers the opening and a cover with a cover bail. Newer cars have a polymer reservoir topped with a plastic cap with rubber seal.

Piston, Seals and Return Ring

The pressure from the master cylinder piston, which is usually made out of aluminum, is regulated by the master cylinder seals. These seals, also called cup seals, act as one-way valves according to Walker, to "capture pressure downstream of the piston during use, but allowing for rapid return flow to the master cylinder when the brakes are released." The return spring forces the piston back to its original position when the brakes are released.

Dual and Triple Master Cylinders

Dual master cylinders are used only on race cars and are arranged side-by-side. One cylinder controls the front brakes, while the other controls the back brakes. This configuration requires the installation of a racing brake pedal.
Triple master cylinders are also used on race cars. According to Walker, "the concept is that brake line pressure cannot only be adjusted between the front brakes and the rear brakes, but also independently between the left front and right front wheels to help the vehicle turn while braking." This is useful because race cars go fast, turn left and repeat.

Optimizing the Master Cylinder

Many owners change the size of the master cylinder pistons to get more control over braking capability. According to Puhn, a smaller diameter piston will reduce pedal effort. Alternately, Walker states that installing a larger diameter master cylinder will not necessarily increase braking power due to the fact that a bigger piston requires more force to push it.
Other optimization techniques include adding a balance bar attached by a balance bar to dual master cylinder configuration. The balance bar, according to Puhn, "proportions the force from the brake pedal to the two master cylinders." Using dual cylinders allows for greater freedom in determining piston size in each cylinder, and allows the driver to customize his braking experience.