A proportioning valve is a device used to distribute fluids at differing pressures from a common source. Commonly machined from a block of steel or aluminum, the proportioning valve uses many channels, springs and diaphragms to modulate the fluid pressure that is released from its chambers. Typically used to apply the proper pressure to a braking system, the proportioning valve was originally comprised of as many as three separate and individual components. The modern proportioning valve is a single piece used to divide the line pressure between the front and rear braking systems of an automobile.
In early American automobile braking systems, both the front and rear brakes were drum units. This does not require a proportioning valve since both drum brake systems utilize the same pressure. The Ford Motor Company was the first to include front disk brakes to its new car lineup in 1966. General Motors and Chrysler followed suit with their 1967 lineup, and the project to develop a simple and reliable proportioning valve was underway. The first attempts at designing a proportioning valve consisted of a multiple component piece, with all components required in order for the unit to perform correctly.
The early brake systems used a dual-reservoir master cylinder, with one side feeding the front brake system and the other serving the rear braking system. This design proved to serve a dual purpose, as well as providing different brake line pressure for the front disk brakes and the rear drum brakes. With the dual reservoir master cylinder, if one brake line should fail, the other brakes could remain in operation to stop the vehicle. Eventually, as plastic components replaced cast iron, a single reservoir master cylinder was used by all automobile manufacturers and the proportioning valve became a necessity on all new vehicles.
With the majority of stopping power on any vehicle coming from the front brakes, the proportioning valve distributes more pressure to the front brakes than the rear when the brake pedal is depressed. In this configuration, the braking is controlled and the vehicle is brought to a stop while remaining in control at all times. If not for the reduction in pressure to the rear brakes, the vehicle could easily spin out of control as the rear wheels would lock up. The amount of pressure drop engineered into the valve is calculated using brake rotor size, tire size and vehicle weight, among various other critical factors.