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A brake combination valve is an essential tool that helps ensure safe operation of a vehicle’s braking system in all conditions.



What Is a Combination Valve?

A car’s braking system works most effectively when braking pressure is applied to the front and rear systems, especially in a situation where the vehicle is equipped with disc brakes on the front and drum backs in the rear. Safe control of the vehicle needs to be maintained in all braking situations and this cannot be easily achieved in the standard system, unless a specific component known as a proportioning or combination valve is fitted.

How Does a Combination Valve Work?

A combination valve has several different functions according to the specific make, model and configuration of vehicle. A proportioning valve will control how much braking force is applied to the rear brakes of the vehicle and is designed to act as a safety measure whenever heavy pressure is applied in an emergency braking situation. The valve restricts how much pressure is applied to the rear brakes, without this feature the rear system would lock up and cause the car to go out of control.

If the car is equipped with a disc and drum combination, then a metering valve is also needed to regulate how much pressure is applied to the front wheels. As disc brakes (fitted to the front) are typically more efficient, then they will apply far more quickly in a heavy braking situation and this could cause the front of the vehicle to become unstable. Typically, a simple switch is also included within the valve housing, which can be activated should brake pressure be lost. When this happens, a light is illuminated on the dashboard instrument cluster to advise the driver that there is a problem with the braking system.

How Is a Combination Valve Made?

A combination valve is incorporated within a machined housing made from brass. Internal components include a diaphragm, several seals and a proportionator piston, which are assembled as one piece and incorporated into the valve before shipment.

How Does a Combination Valve Fail?

  • Internal seals could fail causing a loss of pressure
  • Internal springs could rust or fail

What Are the Symptoms of Failure?

  • Brake pedal could go soft
  • Rear wheels could lock up

What Are the Consequences of Failure?

  • inability to operate the car safely

It’s important to remember the existence of this valve, if you ever have to bleed the braking system on your vehicle. A specific tool can be used that is typically placed into the valve through the warning light socket and which will allow proper bleeding of the system without valve problems.

If you have questions or concerns about Combination Valves or any of your vehicle’s components, come into your local Pep Boys where we can answer any question, help you find any part, or perform any vehicle service you might need.