Relay valve is an air-operated valve. It is used in air brake systems to control the brakes at the rear of a trailer in a tractor-trailer truck. It is used on long wheelbase vehicles to apply and release rear axle's service or parking brakes. An air supply is stored at the rear of the trailer. When the driver presses the brake pedal a small amount of air opens the relay valve in which it directs air from the local source to the rear brakes. It is available in remote and reservoir mount designs. It features inlet valve cartridge replacement without line removal. A relay valve is analogous to a pneumatic transistor.