What is Truck Relay Valve

A truck relay valve is a device that helps reduce brake lag in a vehicle's air brake system. Usually located near the rear service brake chambers, a relay valve collects the signals from the driver when he presses the treadle or pedal, and then sends air from the remote air supply to the rear service brake chambers. This shortens the time required to apply the brakes and allows the compressed air in the air reservoir to fill the chambers quickly.

The relay valve is normally located underneath the vehicle frame. Its function is similar to the transistor in an electronic circuit. When the driver presses the brake pedal, a control line from the foot-valve connects to the relay valve. As soon as the relay is activated, the air is sent from the remote air supply to the brake chambers, accelerating the inflation process.

In dual-circuit air brake systems, air from both the primary and secondary reservoirs is fed into the relay valve's supply port. It then merges through a two-way check valve. This ensures that no overlapping forces can occur in the spring brake chamber, and it prevents air from being lost from only one braking circuit.

In spring brake relay valves, the same principles that apply to a service brake relay valve are applied. However, the difference is that the relay valve's exhaust port is open momentarily. If the brake pressure at the control port drops dramatically, the spring brakes will respond by opening the exhaust port. Once the output of the foot-valve is combined with the output of the parking-control valve, the spring brakes can be controlled with the help of a relay valve.

Compared to the service brake relay valve, a spring brake relay valve is used when the pressure at the control port decreases dramatically. Using the same principle as a service brake relay valve, the difference is that the relief valve opens gradually and bypasses the fluid whenever it is not needed.

Another application for a relay valve is to release the service brakes in case of a breakaway. In this case, the relay is paired with a quick-release valve. Normally, a pilot is used to prevent reverse flow until the pilot pressure is used.

Differential relay valves are generally used in truck brake systems. These prevent overlapping forces in the spring brake chamber from causing brake lag. For a semi-trailer, the service brakes are typically about 60 feet from the brake controls. Relay valves are positioned to shorten the distance between the brake chambers and the foot-valve.

The use of a relay valve in a truck brake system shortens the reaction time, reduces the settling time for the brake pressure, and also improves the overall braking performance. While the dependability of a braking system is dependent on the performance of the driver and the quality of the lines and components, the relay valve can be a major factor.

Generally, the relay valve is installed at the end of the long brake line. This can be at the front of the vehicle or the back. In long wheelbase vehicles, a relay valve is used to correct brake lag.