How to Diagnose Brake Proportioning Valve Failureby Chris Stevenson
The brake proportioning valve on an automotive vehicle serves a very important function in the braking system. Vehicles with front disc and rear drum brakes have proportioning valves located on the brake line between the front and rear brakes. A metering system within the valve, comprised of a small and large valve and a spring, govern the pressure between the front and rear brakes during sudden and emergency brake application. The valve restricts brake fluid pressure to the rear shoes to avoid lockup. Diagnosing a failing or defective proportioning valve requires the vehicle owner to look at a few specific components and observe any unusual braking performance.
Release the emergency brake completely. Drive the vehicle as usual and make several stops, both easy stops and firm ones. If the brake light illuminates on the dashboard, pull over when safe to do so and unsnap the master cylinder fluid reservoir cap located near the brake booster. Fill the reservoir with brake fluid to the upper limit and replace the cap. Continue to drive and note any sponginess in the brake pedal that activates the dash light -- this usually indicates air in the brake lines and is unrelated to the proportioning valve.
Drive the vehicle and perform several medium and hard stops after you have filled the master cylinder. Use an area, such as a parking lot, to perform these maneuvers. Have an assistant watch you while you firmly apply the brakes several times, simulating an emergency panic stop. If the rear wheels lock up for any reason while the front wheels turn and brake normally, you can assume that the proportioning valve is not functioning to relieve pressure on the rear brakes.
Shift the vehicle into "Park" for an automatic or "Neutral" for a standard transmission. Disengage the emergency brake. Lift the vehicle with a floor jack and place two jack stands under the front part of the frame and two jack stands under the rear frame. For older vehicles, the brake proportioning valve will be located underneath the vehicle, usually attached to the side chassis frame or to a floor panel support bracket. On newer vehicles, look for the proportioning valve near the master cylinder. It might sit to the side or underneath the master cylinder and will have an intake line from the master cylinder and an outlet line that splits into a T block that routes lines to the front and rear brakes. Look for leaks at the intake (engine side) brake line where it connects to the valve and the rear brake line that exits the valve. Tighten the flare nuts with a brake line wrench.
Verify that the brake sensor wire has a tight, clean connection to the sensor tip. Examine the proportioning valve body for any leaks, indicating a worn or blown seal. Remove the small rubber mushroom device that sits on the cap screw on top of the proportioning valve. If the interior is rusted, it means the piston could be seized in the cap screw, allowing water to corrode the inside of the valve.
Insert a paperclip inside the small vent hole on the cap screw and exert downward pressure on it. Have an assistant start the engine and apply the brakes very hard. The paperclip should push upward from 1/32 to 1/16 of a inch as a result of the sudden pressure. If the paperclip does not move at all, this means the proportioning valve piston is frozen stuck inside the cap screw. Replace the proportioning valve.
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