The auto proportioning valve, equally known as the combination valve is found in the disc braking system. It is saddled with the duty of 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:
Vehicle rear wheels lock up
Since the proportioning valve lowers the pressure sent to the rear brakes, the main sign the valve is going bad are 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 though gently. As soon as you know the rear wheels are locking up, contact an expert mechanic to have your proportioning/combination valve changed.
How to inspect the problem
If you equally feel you have a problem with the proportioning valve and want to test it by yourself, there is a way to safely do this. Find an empty parking lot and have someone come with you so they can keep an eye on the back brakes. With the person safely away from the car, drive the vehicle and make a few medium to hard stops. Ensure to press the brake many times to simulate an emergency stop. If the rear wheels lock up and the front wheels spins normally, there is an issue with the proportioning valve. This test should be carried out 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 used by technicians to accurately discover and fix failures that may not otherwise be noticed. The brakes are a vital part of your vehicle, and if you need to use them in an emergency, it is vital they work correctly.
Soon as you notice your rear wheels are locking up, particularly on wet surfaces; have your brake system inspected by a qualified mechanic. The proportioning/conditioning valve is an important component of the braking system, so as soon as you see the symptom above, use caution if you need to drive the car.