A master cylinder is a device that converts the motion of a lever or pedal into hydraulic pressure. Master cylinders are commonly used to control a vehicle's clutch or brake systems and can be found on any automobile or motorcycle equipped with hydraulic brakes.
A typical master cylinder is composed of a fluid reservoir, a piston, and the cylinder itself. Pressure is created when the piston is closed, compressing fluid through a valve, or series of valves, in the cylinder and is routed to the brake or clutch systems through pressurized lines. Automotive brake master cylinders often employ a brake booster to amplify the brake pedal pressure to the master cylinder, generating a stronger braking response in return. When used in a brake system application, the master cylinder is attached to brake calipers. Clutch applications employ slave cylinders.
Most modern automobiles use a fail-safe combination valve-type master cylinder system, meaning that a single master cylinder controls two separate circuits by using two valves driven by a two-stage piston setup. With each circuit controlling two of the four wheels, this combination setup allows the brakes to operate even in the event of a malfunctioning brake circuit.
As you depress the brake pedal, the primary piston in the master cylinder compresses the brake fluid into the valve. This pressure continues and forces the secondary valve to close, sending the brake fluid from the cylinder into the brake lines to close the brake calipers attached to both circuits. Should a leak occur in the master cylinder, the pressure between the primary and secondary pistons is lost. However, the primary piston presses against the secondary piston and act as a single-piston setup, maintaining brake functionality in an emergency.
Motorcycles use a master cylinder to power their brake and clutch systems as well. Typically much smaller than their automotive cousins, they operate on the same hydraulic principles. Front brake cylinders operate on a single valve that actuates up to two calipers on the front wheel. Rear brake master cylinders are smaller in design and often less powerful to prevent locking of the rear wheel. Radially mounted master cylinders, which are used mainly on newer high-performance motorcycles, mount the master cylinder perpendicularly to the handlebar, and are claimed to increase braking power and transmit improved brake feedback to the rider.
Clutch master cylinders operate in the same manner as a brake master cylinder, converting clutch pedal pressure into hydraulic pressure to manipulate a slave cylinder, disengaging the clutch. Automotive-type clutch master cylinders are typically mounted next to the brake master cylinder with the clutch slave cylinder mounted on the transmission clutch bell housing. Motorcycle clutch master cylinders are located on the left handlebar and are actuated when the clutch lever is pulled in.
Brake fluid absorbs moisture and degrades over time leading to corrosion build-up, in the form of rust, within the master cylinder. It is recommended that you flush your brake and/or clutch lines every to two years. More frequent fluid changes may be required for high-performance machines. If the fluid is cloudy or is filled with debris, replace the fluid immediately. Bleed the brake (or clutch) lines, being sure that any air trapped in the line is removed to prevent loss of hydraulic pressure.
