The master cylinder is a foot operated hydraulic pump that sends pressurized brake fluid through the brake lines and into the brake calipers/wheel cylinders.
The master cylinder usually consists of a housing, reservoir, piston, rubber cup, return spring and a rubber boot (manual only). A cylinder bore is machined into the center of the housing. The spring, cup and piston(s) slide in this bore and produce hydraulic pressure. The reservoir keeps the system full of fluid as the brake lining wears and the dust boot serves to keep contaminates from entering the rear of the master cylinder.
The dual master cylinder is required on all vehicles manufactured since 1967. Dual master cylinders have two separate chambers that separate the front and rear brake circuits. This type of system prevents the total loss of braking action in the event of brake fluid loss. The brake circuits can be split front and back or diagonally. Both styles will stop the vehicle with only one circuit operating, but it is not safe for normal use.
Operation of the master cylinder is simple. When the brake pedal is depressed, force is applied through the push rod to the master cylinder piston. As the piston cup is forced forward, brake fluid is trapped and hydraulic pressure builds. Hydraulic pressure causes the movement of brake assemblies, such as calipers or wheel cylinders, to stop the rotation of the wheels. When the brake pedal is released, fluid is forced back through the lines into the master cylinder reservoir.