This document is applicable to relay valves designated to control the brake cylinder pressure of compressed air brakes fitted to railway vehicles, in association with an air brake distributor valve or other control device. It covers one stage relay valves and relay valves adjusting the brake cylinder pressure in response to a change in vehicle speed or load that is either continuously variable or in two or more stages, i.e. empty – loaded. Relay valves operating with other pressures, in particular the brake pipe pressure, are not included. This document specifies the requirements for the design, manufacture and testing of relay valves.