A brake proportioning valve serves a very important function on an automotive vehicle braking system. The proportioning valve resembles a small metal block which has an inlet and outlet brake line fitting, which connects the front brake calipers with the rear brake calipers, or wheel cylinders. The proportioning valve functions as a pressure equalizer for the hydraulic brake fluid, making sure the pressure applied to the front brakes matches the rear brakes. The result provides adequate and monitored pressure to allow even braking over all wheels on the vehicle. Priming the proportioning valves requires a routine bleed of the system.
Instructions
- 1
Place the vehicle in "Park" or "Neutral" and apply the emergency brake. Use a floor jack to raise the front of the vehicle and place two jack stands under the frame. Do the same with the rear part of the vehicle. Locate the proportioning valve underneath the chassis -- it should be located midway between the front and rear wheels, with brake line extending from both ends of it. If you are unsure of its location, refer to your owner's service manual. Some proportioning valves attach near the master cylinder inside the engine compartment.
2Tighten both line fittings on the proportioning valve with a brake line wrench, to ensure they have no leaks. Do this especially if you have installed a new proportioning valve. Use a rag to wipe off all of the bleeder valves (nipples) on all four wheels. The bleeder valves attach directly to the front calipers, and the rear valves protrude through the backing plate on the rears. You might have calipers (disk brakes) on the rear. Open the hood and unsnap or unscrew the master cylinder cap. Fill the master cylinder with brake fluid and replace the cap.
3Locate the rear bleeder valve furthest from the master cylinder location -- usually the right rear. Attach a clear plastic hose to the nipple part of the brake bleeder and lead it down into a drain pan or container. Instruct an assistant to man the driver's seat and release the emergency brake. Have the assistant pump the brake pedal two to three times and firmly hold it.
4Place a brake line wrench on the bleeder valve and turn it slightly counterclockwise. Allow the air and bubbles to escape. Tighten the bleeder valve. Have your assistant repeat the pump and hold maneuver. Open the bleeder valve again and let all air out, until a steady stream of brake fluid exits. Close the bleeder valve.
5Move to the opposite rear wheel, transferring your hose and drain pan. Instruct your assistant to pump two to three times and firmly hold. Open the bleeder valve and let all air pass out. Repeat the process until clear brake fluid exits the hose. This will prime and "set" the pressure inside the proportioning valve. Move to the front of the vehicle and connect the drain hose to the furthest bleeder valve from the master cylinder. Check your brake fluid master cylinder level, and add fluid as needed. Bleed both front brakes in the identical fashion you bled the rears.
6Fill the master cylinder with brake fluid to the prescribed top level, when you have finished the bleed on all four wheels. Replace the master cylinder cap. Dispose of your old brake fluid and drain hose. Use the floor jack to lift the vehicle and remove all four jack stands. Start the engine and test the brake response. The pedal should be firm, with no spongy feel.
