How to Install and Plumb a Brake Proportioning Valve

Confirm the part and repair scope

First, confirm that the valve is actually the problem. Rear wheels locking early, a vehicle that becomes unstable under braking, or a brake warning light can point to a failed proportioning or combination valve. The same symptoms can also come from contaminated friction material, mismatched brake pads, a seized caliper, brake rotors or drums worn past their wear limits, or a fluid leak. Inspect those components first, because a new valve will not cure a mechanical problem at the wheel. Many late-model vehicles have no separate valve at all. Their ABS module manages rear pressure through electronic brake force distribution, and adding a valve without engineering support can upset that system.

Next, identify which type of valve your vehicle uses. Factory systems often put the proportioning function, a metering valve and a pressure differential warning switch into one combination valve, usually mounted near the master cylinder. Some trucks and vans use a load-sensing valve linked to the rear axle, which changes rear pressure with ride height. Aftermarket adjustable valves are common on disc brake conversions and custom builds. Check vehicle fitment by comparing part numbers with the original, and match the port thread sizes and flare type. If you are adding a valve to a modified system, make sure it suits your master cylinder and front-to-rear brake layout.

Prepare tools and safe access

You will need flare nut (line) wrenches in the correct sizes and fresh brake fluid of the type specified on the reservoir cap or in the owner's manual. You will also need a drain pan, rubber caps or line plugs, shop rags, and a bleeding method you are comfortable with. If new lines are needed, add a tube cutter, a tubing bender and a flaring tool that makes the correct flare. Raise the vehicle on level ground and support it on rated jack stands. Chock the wheels that stay on the ground, and never work under a car held up only by a jack.

Get the vehicle-specific service procedure before you begin. It gives the fitting torque, any special bleeding steps for a combination valve, and setup instructions for load-sensing valves. Brake fluid strips paint and absorbs moisture, so cover nearby painted surfaces and keep fluid containers sealed. Consider having a qualified brake technician do the work if you have not serviced hydraulic brake lines before, or if the job requires new flares or adjusting a load-sensing valve. A leak or trapped air can cause a sudden loss of braking.

Remove without avoidable damage

Before you disconnect anything, photograph or label each line so you know which port it came from. On a combination valve, the lines from the master cylinder and the lines to the front and rear brakes sit close together and are easy to swap. Unplug the warning switch connector if your valve has one. To limit fluid loss, place plastic film under the reservoir cap or use a brake pedal depressor. Loosen each fitting with a flare nut wrench, because open-end wrenches tend to round off the soft fitting nuts.

Fittings that have been in place for years are often seized. Apply penetrating oil and give it time to work. Hold the valve body steady while you turn each nut so the steel lines do not twist. If a line starts to turn with its nut, stop, because a twisted line must be replaced. Cap each line and port as soon as it is open to keep out dirt and moisture. Then remove the mounting bolts and lift out the valve. Check the removed lines for corrosion, kinks and damaged flares, and replace any questionable line rather than reusing it.

Install and restore the system

Mount the new valve in the same orientation as the original, or as the valve manufacturer directs. Secure it with its bracket so it cannot vibrate against the body or other lines, and keep it away from exhaust heat and moving suspension parts. On a custom or converted system, plumb an add-on valve into the rear brake circuit only. The line from the master cylinder's rear outlet goes to the valve inlet, and the valve outlet feeds the rear brakes. Most aftermarket valves mark the inlet or show a flow direction. A valve installed backwards will not work correctly.

Start every fitting by hand to avoid cross-threading, then tighten it with a flare nut wrench to the specified torque. Flared brake fittings seal at the flare, so do not use thread tape or sealant. Double (inverted) flares go only into inverted flare ports, and bubble flares go only into bubble ports. Mixing the two will cause a leak. Bend any new tubing smoothly with a bender, and leave a small loop for movement wherever the factory had one. Finally, reconnect the warning switch connector.

Fill the reservoir and bleed the entire system in the sequence given in the service information. The sequence commonly starts at the wheel farthest from the master cylinder. Some combination valves need the metering valve held open with a special tool during pressure bleeding, and some warning switches must be recentered afterward, so follow the specific instructions. Load-sensing valves usually must be adjusted with the vehicle at a specified ride height or load. For an adjustable valve, start at the setting that delivers the most rear pressure unless the manufacturer says otherwise.

Check the completed work

With the vehicle still raised, have a helper press the pedal firmly while you check every connection for weeping fluid. Lower the vehicle and top up the reservoir to the correct level. The pedal should be firm and high both with the engine off and with it running, and the brake warning light should be off. A soft or sinking pedal, a lit warning light or any visible leak means the car is not ready to drive. Recheck the bleeding and the fittings before going further.

Then test at low speed in a safe, empty area, working up gradually to firmer stops. The vehicle should stop straight without pulling, and the rear wheels should not lock before the fronts. If you installed an adjustable valve, make small changes to rear pressure between test stops until the fronts lock just before the rears, then lock the adjuster in place. Recheck the fittings and fluid level after the first few drives. If rear lockup will not tune out, the car pulls to one side or the warning light stays on, there is a further fault in the hydraulics, calipers or wheel brakes. Have it diagnosed professionally before returning to normal use.