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Learn How Your Master Cylinder Operates

Master Cylinder Location

Diagram of master cylinder

When the brake pedal is depressed, it pushes on the first (primary piston) through a linkage. The Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. When the brakes are operating correctly, the pressure will be the same in both circuits.

Learn How The Proportioning Valve Operates


A Proportioning valve is required on vehicles that have disc brakes on the front wheels and drum brakes on the rear wheels. Disc brake pads are normally in contact with the disc, while the drum brake shoes are normally not in contact with the drum. If the pressure was not proportioned the disc brakes would engage before the drum brakes when you depress the brake pedal.

The Proportioning valve compensates for this, allowing the drum brakes to engage first before the disc brakes. The Proportioning valve does not allow any pressure to the disc brakes until a pre-determined pressure has been reached. The pre-determined pressure is low when compared to the maximum pressure in the braking system, this allows the drum brakes to engage before the disc brakes engage. Having the rear brakes engage first provides the control and stability needed to stop your vehicle safety.

The proportioning valve reduces the pressure to the rear brakes. Whichever type of brakes your vehicle has, the rear brakes require less pressure than the front brakes.

If equal braking force were applied to all four wheels during a stop, the rear wheels would lock-up before the front wheels. The proportioning valve only lets a portion of the amount of pressure to the rear wheels thus preventing rear wheel lock-up.