The proportioning valve, also called the combination valve, is located in the disc braking system. It is responsible for controlling the braking pressure between the front and rear brakes. Equal brake force cannot be applied to all of the wheels on the car at the same time because the rear wheels will lock up. Vehicles with drum brakes in the rear have much lower pressure for the drum brakes than the front disc brakes. This is why the proportioning valve only lets a particular amount of pressure through to the rear brakes. If you suspect there is an issue with your proportioning valve, check for the following symptom:
Rear wheels lock up
Since the proportioning valve decreases the pressure sent to the rear brakes, the main symptom the valve is going bad is the rear wheels locking up when the brakes are applied. Furthermore, the wheels will lock up more easily on wet surfaces. The rear brakes may feel touchy when applied even gently. As soon as you notice the rear wheels locking up, contact a professional mechanic to have your proportioning/combination valve replaced.
How to check the problem
If you suspect you have a problem with the proportioning valve and want to test it on your own, there is a way to safely do this. Find an empty parking lot and have someone come with you so they can watch the back brakes. With the person safely away from the vehicle, drive the car and make a few medium to hard stops. Make sure to press the brake several times to simulate an emergency stop. If the rear wheels lock up and the front wheels turn normally, there is an issue with the proportioning valve. This test should be performed with extreme caution.
Best left to the professionals
Repairing and replacing the proportioning/conditioning valve is best left to the professionals because you are dealing with the braking system. While you can test the problem on your own, it is a good idea to have a professional mechanic fix the problem. Specialized tools are used by technicians to accurately diagnose failures that may not otherwise be noticed. The brakes are an essential part of your vehicle, and if you need to use them in an emergency, it is important they work correctly.
As soon as you notice your rear wheels are locking up, especially on wet surfaces, have your brake system checked by a qualified mechanic. The proportioning/conditioning valve is an important part of the braking system, so as soon as you see the symptom above, use caution if you have to drive the vehicle. YourMechanic makes getting repairs to your proportioning / combination valve easy by coming to your home or office to diagnose or fix issues. You can book a service online 24/7. YourMechanic’s qualified technicians are also available for any questions that may arise.
This article originally appeared on YourMechanic.com as Symptoms of a Bad or Failing Proportioning/Combination Valve and was authored by Spencer Clayton.