Brake Proportioning Valve Operation

The proportioning valve is installed on vehicles with front disc rear drum braking systems. It is installed in the hydraulic line leading to the rear drum brakes and helps prevent rear wheel lock up. Many proportioning valves are mounted directly in the master cylinder outlet to the rear brakes. They provide balanced braking during panic braking situations or when the vehicle is loaded by restricting fluid pressure to the rear brakes. This helps prevent rear wheel lock up as the vehicles weight is shifted toward the front wheels.

The typical inline proportioning valve remains open until a panic brake situation occurs. When sudden high pressure from the master cylinder moves the larger piston it closes the valve restricting fluid pressure.

In late model vehicles these valves have been incorporated into the combination valve along with the metering valve and failure warning light switch. As mentioned earlier they are often threaded into the master cylinder outlet ports to the rear lines.

Another common proportioning valve is the load sensing proportioning valve. The load sensing proportioning valve is usually found on trucks and is located between the vehicles chassis and the rear axle. When a driver brakes hard in a pickup truck the rear end tends to lift up and forward. This lightens the load on the rear tires increasing the chance of lock up. This valve incorporates a lever attached to a spring that moves the valve to restrict fluid pressure as the vehicles chassis raises away from the axle.

An electronic brake proportioning EBP has replaced the conventional proportioning valve in many late model vehicles. This system monitors the speed difference between the front and rear wheels and utilizes the ABS system to prevent rear wheel lock-up.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Nathaniel Olson says:

    Hello, wondering if a torque wrench can be used to check combination valve operation if it is on a non abs vehicle.

    • Martin Hand Martin Hand says:

      I don’t know how you would use a torque wrench to test proportioning valve they either work or they don’t there is nothing to torque on it anyways. I just replace it if questionable, should be fairly in-expensive.

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