A proportioning valve is a part of your vehicle’s braking system. It takes the fluid pressure supplied by the master cylinder and reduces it to the rear brakes. The front brakes of any vehicle do the majority of braking, therefore the rear brakes don't require as much stopping power.
An equal distribution of braking power to the front and rear brakes would cause the rear brakes to lock up and create an unsafe condition. Any air introduced in the brake lines will create softness in the brake pedal, causing it to travel all the way to the floor, preventing proper and safe braking. Bleeding the air out of the brake lines and proportioning valve will prevent this issue.
Tools and Materials
- Floor jack
- Lug nut wrench
- Open-end/box wrench
- Drip pan
- Brake fluid
- A helper
Step 1 – Preparation
The proportioning valve doesn't have any way to bleed air, so you will have to bleed the front and rear brake lines in order to remove any air trapped in the valve. Park your car on a flat surface and set the emergency brake, to prevent your car from rolling while you are working under it. You want to start bleeding the brake lines at the location farthest from the master cylinder, so you will raise your vehicle using the floor jack at the right rear tire location first. Loosen the lug nuts for this tire and then raise the vehicle using the floor jack. With the tire off the ground, finish removing the lug nuts and remove the tire from the wheel hub.
Step 2 – Bleeding the Brake Lines
Locate the brake line leading to the brake caliper or drum brake assembly, and place the drip pan under the wheel. Take an open-end wrench and slip it on the brake bleeder screw. You will need a helper to push the brake pedal. However, do not let your helper sit in the vehicle. Have them use their hand from outside the vehicle. Ask your helper to apply slow, constant pressure to the brake pedal. Turn the screw counter clockwise slowly, until you see air bubbles escaping from the bleeder valve. When your helper tells you the brake pedal is to the floor, close the bleeder valve, to prevent air from entering the brake line, as the pedal is brought back to the up position. Repeat these procedures until all the air is out of the brake line. Tighten the brake bleeder valve completely and replace the tire on the wheel hub. Remove the drip pan from under the wheel and slowly lower the car back to the ground. Finish tightening the lug nuts on the tire.
Step 3 – Finishing Procedures
Finish bleeding the proportioning valve by repeating the above procedure on the remaining three brake lines. Replace any brake fluid that was pushed out of the brake lines, and dispose of the used fluid per local guidelines. Once all the brake lines have been bled, there should be no air left in the proportioning valve. If you find there is still air in the lines or the valve, you will have to inspect the brake lines, master cylinder and proportioning valve for leaks, and replace whichever parts are faulty.