How to Get a Proportioning Valve Unstuck

by Chris Stevenson
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The brake proportioning valve is a metering device that equalizes the pressure between the front and rear brakes. The valve stops the flow, or pressure of brake fluid to the rear drum brakes during every heavy braking or emergency stops; otherwise, the rear brakes lock up and skid, receiving too much pressure from the master cylinder. A spool or stem slides back and forth inside the piston bore of the proportioning valve, according to the pressure it receives. Proportioning valves can get stuck at either end of the valve body, requiring a procedure to free them up.

Step 1

Place the vehicle in park, for an automatic transmission, and in neutral for a standard. Apply the emergency brake and lift the hood. Unscrew, or unsnap the master cylinder cap and fill the master cylinder with brake fluid to the top limit line. Leave the cap off. Use a floor jack to raise the vehicle at the front end and place two jack stands under the frame near each wheel. Raise the rear of the vehicle and place two jack stand under the rear frame near each wheel.

Step 2

Depress the brake pedal and note if you have a brake light on the dashboard. If your brake system has not been previously worked on or suffered no brake leaks from the wheel cylinders, calipers or master cylinder, chances are the brake proportioning valve is stuck in one position, tripping the brake dash light. If your brake system has suffered a leak or been repaired recently, you will need to perform a standard brake bleed.

Step 3

Have an assistant sit in the driver's seat. Place a drain pan under the right rear wheel of the vehicle. Place a flare nut wrench on the brake bleeder valve, the small nut with the round opening, exiting the backing plate. Have your assistant pump the pedal three or four times and hold it. Loosen the brake bleeder with the flare nut wrench and allow brake fluid to exit. If you see air and bubbles, repeat the process again until a clear stream of brake fluid exits the valve.

Step 4

Perform the same brake fluid bleed on the rest of the wheels, continuing next with the left rear wheel, until all air is removed. Fill your master cylinder with brake fluid. Bleed the right front bleeder valve, then bleed the left front bleeder valve last. Ask the assistant if the dash light has gone out. If not, you will need to check the proportioning valve, which will be located next to your master cylinder. The valve looks like a small cast iron block, and it has a master cylinder line feeding it, with rear and front brake lines attached to it.

Step 5

Tap the proportioning valve with a tack hammer several times lightly -- sometimes this frees up a corroded bore and piston spool inside the valve. Wiggle the brake light warning switch wire on the top of the proportioning valve, making sure it has a tight, clean connection. Locate the brake line on the valve that leads to the rear brakes, as well as the two single lines that lead to the left and right front brake calipers.

Step 6

Place a flare nut wrench on the rear cap nut to the rear brake line. Instruct your assistant to hold the brake firmly, while you open the cap nut with the wrench. If the brake pedal does not move downward, have your assistant stomp on the brake pedal with medium to hard pressure, and hold it down firmly. If you hear a click from the valve, and your assistant tells you the dash light has gone out, stop and close the valve. The interior spool device inside the proportioning valve has moved and centered.

Step 7

Have your assistant hold the brake pedal down while you place the flare nut wrench on one of the front brake line cap nuts on the proportioning valve, if the dash light remained on. Loosen the cap nut with the wrench and have your assistant firmly press the brake pedal. Wait for fluid to exit, and have your assistant alert you if the light goes out. If not, have your assistant use the stomping push, and listen for the click. Close the cap nut. Perform the same procedure on the remaining front brake line, opening the line, foot on the brake and some quick stomps.

Step 8

Bleed the rear brake line again at the proportioning valve, as you did before if you still have a brake dash light. Bleed the front brake lines on the proportioning valve in the same fashion. This reversal of pressure will eventually break the spool loose inside the proportioning valve, equalizing the pressure between the front and rear brakes. The dash light will go off.

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