The master cylinder consists of a reservoir and one or two piston assemblies. Dual circuit master cylinders are designed to hydraulically separate the front and rear brake systems in the event of a brake fluid leak. The master cylinder converts mechanical pedal pressure into hydraulic pressure within the line. This pressure is converted back into mechanical pressure by either the wheel cylinders (drum brakes) or the calipers (disc brakes). Since they receive pressure from the master cylinder, they are classified as slave cylinders.
The brake fluid is supplied to the appropriate places on the vehicle frame near the wheels via steel pipes, and then to the actuator cylinders via flexible tubes, which allows the suspension and steering components to move freely.
Each wheel cylinder contains two pistons, one at each end, which act in opposite directions to force the brake shoe into contact with the drum. In disc brake systems, the slave cylinders are part of the calipers. One, two or four cylinders are used, but all contain only one piston. All pistons use some type of
a seal (usually rubber) to reduce fluid leakage around the piston. A rubber dust boot protects the cylinder from dust and dirt. The boot fits over the protruding end of the piston or the brake actuator lever.
At rest, the entire hydraulic system, from the pistons in the master cylinder to the pistons in the wheel cylinders or calipers, is filled with brake fluid. Under the action of the brake pedal, the fluid, carried by the piston(s) of the master cylinder, is directed through pipes to the slave cylinders, in which it sets the pistons in motion - outward, in the case of drum brakes, or inward (towards the disc) in the case of disc brakes. The movement of the pistons is balanced by return springs mounted outside the cylinders (drum brakes) and internal springs or spring seals (disc brakes).
When the brake pedal is released, a spring inside the master cylinder immediately returns the master cylinder piston(s) to their original position. The pistons contain check valves, and the master cylinder has compensation passages drilled inside it. These open when the pistons return to their original position. The piston check valves allow fluid to flow to the wheel cylinders or calipers when the pistons return. Then, as soon as the return springs return the brake shoes to their original position, excess fluid in the lines causes the replenishment reservoir to fill through the compensation passages.
The master cylinders of the diagonally split brake systems use two pistons arranged coaxially one behind the other in a single cylinder. The primary piston is actuated directly by a mechanical linkage from the brake pedal. The secondary piston is actuated by the fluid between the pistons. If a brake fluid leak occurs in front of the secondary piston, it moves forward until it reaches the front wall of the master cylinder, and the fluid between the pistons actuates the rear brakes. If a leak occurs in the rear brakes, the primary piston will move forward until it makes direct contact with the secondary piston, which actuates the front brakes. In either case, the brake pedal has further travel and a small braking force is usually sufficient.
All dual-circuit brake systems have a warning light that is sensitive to either line pressure or fluid level. This system will alert the driver if half of the brake system fails.
Some disc brake systems also have a proportioning valve and, in some cases, a distribution valve. The proportioning valve keeps the pressure in the front disc brakes constant until the brake shoes on the rear wheels make contact with the drum, ensuring that the front brakes are never used alone.
The control valve controls the pressure in the rear brakes, preventing the rear wheels from locking in the event of emergency braking.
CAUTION: Brake pads contain asbestos, which is a carcinogen. Never clean brake surfaces with compressed air. Avoid inhaling any dust from the brake surface. When cleaning brakes, use special brake cleaning fluids.