The proportioning valve, equally referred to as the combination valve is found in the disc braking system. It is equally 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 vehicle at the same time since the rear wheels will lock up. Cars 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 allows a particular amount of pressure through to the rear brakes. If you feel there is an issue with your proportioning valve,inspect for the following symptom:
Rear wheels that lock up
Since the proportioning valve reduces the pressure sent to the rear brakes, the main sign of the valve 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 anytime applied even gently. As soon as you observe the rear wheels locking up, contact a professional mechanic to have your proportioning/combination valve replaced.
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How to check the Issue
If you feel you have a problem with the proportioning valve and want to test it on your own, there is a way to safely perform 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 car, drive the vehicle 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, if there is an issue with the proportioning valve. This test should be performed with extreme caution.
Best allowed to the professionals
Repairing and changing the proportioning/conditioning valve is best left to the professionals since you are dealing with the braking system. While you can test the issue on your own, it is a good idea to have a professional mechanic fix the issue. Specialized tools are made use of by technicians to accurately diagnose failures that may not otherwise be seen. The brakes are an essential part of your car, and if you need to use them in an emergency, it is vital they work correctly.
As soon as you observe your rear wheels are locking up, particularly on wet surfaces, have your brake system inspected by a qualified mechanic. The proportioning/conditioning valve is a vital part of the braking system, so as soon as you observe the symptom above, use caution if you have to drive the car.