Brake Master Cylinder: Description and Operation

Schematic diagram of a typical hydraulic diagonally split brake system:

This system operates on the same principles as conventional front and rear split systems using primary and secondary master cylinders moving simultaneously to exert hydraulic pressure on their respective systems.

The hydraulic brake lines on this system, however, have been diagonally split front to rear (left front to right rear and right front to left rear) in place of separate lines to the front and rear wheels. In the event of a system failure this would cause the remaining good system to do all the braking on one front wheel and the opposite rear wheel, thus maintaining 50% of the total braking force. The hydraulic pressure loss would result in a pressure differential in the system and cause a warning lamp on the dashboard to glow as in front and rear split systems.

Failure warning switch, rear system failure:

Failure Warning Switch
If the rear brake system fails, the front system pressure forces the switch piston to the right. The switch pin is then forced up into the switch, completing the electrical circuit and activates the dash warning lamp.
When repairs are made and pressure returns to the system, the piston moves to the left, resetting the switch. The detent on the piston requires approximately 100 to 450 psi to permit full reset of the piston. In event of front brake system failure, the piston moves to the left and the same sequence of events is followed as for rear system failure except the piston resets to the right.

Proportioner, rapid deceleration:

Proportioner, normal braking:

Proportioners installed in master cylinder:

Proportioner Valves
During rapid deceleration, a portion of vehicle weight is transferred to the front wheels. This resultant loss of weight at rear wheels must be compensated for to avoid early rear wheel skid. The proportioner or pressure control valve reduces rear brake system pressure, delaying rear wheel skid. When the proportioner valve is incorporated in the combination valve assembly, pressure developed within the valve acts against the large end of the piston, overcoming the spring pressure, moving the piston left. The piston then contacts the stem seat and restricts line pressure through the valve.


During normal braking operation, the proportioner valve is not functional. Brake fluid flows into the proportioner valve between the piston center hole and the valve stem, through the stop plate and to the rear brakes. Spring pressure loads the piston during normal braking, causing it to rest against the stop plate.


On diagonally split brake systems, two proportioners are used. One controls the left rear brake, the other the right rear brake. The proportioner valves are installed in the master cylinder rear brake outlet ports.