Relay Valve

A Relay Valve is an air-operated valve. It is most-commonly used in air brake systems to control the brakes at the rear of a trailer in a tractor-trailer truck.

It would take too long for sufficient air to travel from the brake pedal valve to the rear of the trailer to operate the brakes in conjunction with the other brakes. Therefore an air supply is stored at the rear of the trailer whenever the emergency brakes are released. When the driver presses the brake pedal a small amount of air opens the relay valve which directs air from the local source directly to the rear brakes.

A relay valve is analogous to a pneumatic transistor.