What is the Proportioning / Combination Valve all about?
Your braking system relies on hydraulic pressure to operate. When you depress the brake pedal, brake fluid is pushed through the system to the wheels. Fluid pressure is responsible for actuating both disc brake calipers, as well as drum brakes. However, there are numerous components in line between the brake pedal and the brakes themselves, including the combination valve.
The combination valve is responsible for metering fluid to disc brakes, as well as fulfilling tasks done by both the pressure differential switch and proportioning valve (which is responsible for reducing the fluid pressure in the lines leading to the rear drum brakes). This is necessary because of how brakes work in terms of lockup – the more weight there is on a wheel, the more force can be applied without locking up the brakes. In rear drum brakes, the weight on the wheel is comparatively small (the weight of the engine on the front wheels is considerable).
Keep in mind:
- In modern cars that employ combination valves, replacing the entire valve body is necessary if the proportioning valve fails.
- There is no way to check operation on this type of valve unless it has failed completely.
- The valve may have failed due to many reasons, including debris in the brake fluid.
How it's done:
- The defective proportioning combination valve brake lines are disconnected
- The defective proportioning combination valve is removed from the vehicle
- The new proportioning combination valve is installed and the brake lines are connected to valve
- The brake system is bled and purged of air in the system and the brake fluid is refilled
- The vehicle is road tested for proper brake operation
Follow your automaker’s recommendations on brake system service to ensure that the fluid remains in good condition and free of debris. Have your braking system maintained by our expert mechanics to ensure that any potential problems are caught as soon as possible. Driving with a failing proportioning valve puts you at increased risk.
What common symptoms indicate you may need to replace the Proportioning / Combination Valve?
- Rear brakes lock up easily, particularly on wet pavement
How important is this service?
While it is still possible to drive with a failed or failing proportioning valve, it’s not recommended because it decreases your safety while increasing the risk of being involved in an accident because your rear brakes lock up under even light loads. Have the entire combination valve assembly replaced immediately.