What is a Relay Valve?

C. Daw

The relay valve within a vehicle is an air pressure operated device that collects signals from the driver when they apply the brake. It is commonly found on larger vehicles, such as buses and tractor trailers, because of their size. The velocity that is built up behind these vehicles is substantially higher than an average vehicle would be, so in order to stop properly a special braking system must be in place. The relay valve ensures that the rear brakes work with the front ones to guarantee that larger vehicles can stop when needed.

Large vehicles, such as buses, commonly employ relay valves to ensure the vehicles stop properly.
Large vehicles, such as buses, commonly employ relay valves to ensure the vehicles stop properly.

The relay valve is hooked to the brake pedal and the rear brakes through various series of wires, which are wired through a central control unit in newer vehicles. It can usually be found on the frame underneath the vehicle next to the air tank. The air tank stores air next to the brakes so when the relay valve activates, the air is forced into the brakes immediately, rather than having a delay. The valve itself has one inlet, and two or more outlets, depending upon the size of the vehicle. This allows the sensor to relay the braking action from the pedal to the various wheels as soon as the brake pedal is pressed down.

Applying the brake sends signals to the relay valve in a vehicle.
Applying the brake sends signals to the relay valve in a vehicle.

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When the driver presses down on the brake pedal, the front braking system is activated through the basic braking system, and the relay switch sends a signal to the rear brakes through the air tank, to begin working. This allows the braking system to effectively stop the vehicle by applying uniform pressure to the brakes on all the wheels. The way that this works is through commands sent from the brake pedal, to the relay valve, to the air tank, which forces air into the lines and out to the brakes in the rear wheels. This split-second process causes the rear brake to activate along with the front brakes, helping the vehicle to stop safely and quickly.

The dependability of the braking system will center on the driver’s performance, the condition of the brakes and lines, and how well the relay valve works. The main task of the relay valve is to reduce the time taken to brake the rear wheels. If the relay does not function properly, the delay will cause the rear brakes to begin working too late. This delay can end with serious braking problems, including jack-knifing in trucks that haul trailers.

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