THE MASTER CYLINDER

All passenger cars and light trucks in the United States today are equipped with a dual master cylinder. The dual master cylinder, also known as a tandem master cylinder, is essentially two separate master cylinders combined into one unit. The dual master cylinder is used as a safety device, to divide the brake hydraulic system into two individual hydraulic circuits. There are two types of split hydraulic systems used. Most rear wheel drive vehicles use a conventional split system, dividing the brake circuits by axle. One half of the master cylinder is used to operate the front brakes, while the other is used to operate the rear brakes. A large number of front wheel drive vehicles use a split diagonal system. The split diagonal system divides the brake circuits between one front wheel and one rear wheel on opposite sides of the vehicle. In the event of pressure loss in one circuit, the other hydraulic circuit would remain operational. The dual master cylinder has been required equipment by law, since 1967. Some automobiles manufactured before 1967 were required to be retrofitted by a certified auto repair mechanic for a dual master cylinder.

The master cylinder is generally constructed of aluminum or cast iron. Attached to or contained in the master cylinder body itself, are two separate brake fluid reservoirs. Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion. This is to allow for fluid level changes as the disc pads wear. Inside the master cylinder is a machined bore that contains the master cylinder primary and secondary pistons. Both pistons are combined into a single, two part unit. The section that is closest to the brake push rod is the primary piston, while the section closest to the front of the master cylinder is the secondary piston. Cup seals are placed on the primary and secondary pistons to provide a tight seal in the automobile master cylinder bore. Calibrated springs are placed between the primary and secondary piston and the secondary piston and the end of the master cylinder bore. The area in front of each piston is the pressure chamber area and this is where pressure is created to operate the brake system. There are two ports machined into the master cylinder bore for each brake fluid reservoir. One port is the by-pass port, the other is a compensating port. The by-pass port supplies fluid to the master cylinder bore when the brakes are not applied. The compensating port is used to allow the master cylinder piston to quickly return to the unapplied position, as well as compensate for fluid expansion or contraction from changes in temperature.

The master cylinder is operated by a push rod attached to the brake pedal. When the driver depresses the brake pedal to stop the vehicle, the brake pushrod moves the master cylinder primary piston forward in the master cylinder bore. The secondary piston moves at the same time, operated by hydraulic pressure created in front of the primary piston and the calibrated spring placed between the pistons. As the piston moves forward, the by-pass port is blocked by the cup seal, creating a sealed pressure area in front of the piston. Hydraulic pressure is created by the pressure of the piston on the brake fluid. When the brake pedal is released, it is possible for a vacuum to be created behind the piston as it returns to rest position. To prevent this, the compensating port allows fluid to flow into the area behind the brake piston.

In the event of the loss of one hydraulic circuit, the remaining circuit would be operated in part by the failed circuit piston and the brake pushrod. If the primary circuit were to fail, the loss of pressure would cause the primary piston to move forward in the master cylinder bore, until it bottomed against the calibrated spring and the secondary piston. The primary piston would then act as a connector between the brake pushrod and the secondary piston. If the secondary circuit failed, the secondary piston would move forward against the secondary piston spring, until it reached the end of the master cylinder bore. When the secondary piston reaches the limit of its travel, the primary piston is then able to build pressure in the primary brake circuit. Of course, when diagnosing and troubleshooting the automobile master cylinder and overall braking system, both hydraulic circuits must be tested by an auto repair mechanic for 100% functionality.

Most master cylinder malfunctions are the result of failed seals, as any auto repair mechanic will attest to. Leaks can develop at the rear seal of the master cylinder, resulting in fluid loss at the pushrod area. Cup piston seals can harden or wear, causing brake fluid to leak past the brake pistons, resulting in loss of pressure inside the master cylinder. Poor internal sealing due to worn or hardened cup seals is most evident by the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop. If you are not a do-it-yourselfer, make sure you have an auto repair shop inspect your master cylider at least once per year. Generally, the best auto repair facility to have repairs performed is the dealer, although the dealer is usually the most expensive. Independent auto repair shops can be just as reliable, however. Check with your local Chamber Of Commerce or Better Business Bureau to find a reputable auto repair facility.