Abstract: A coupling for connection to a source of fluid pressure such as in a breathing-protection. The coupling has a socket end and a plug end insertable into the socket end. The socket end has a body and a locking sleeve on the body. As the plug end is inserted into the socket end, the locking sleeve is moved axial from a first axial position to a second axial position locking the plug end within the socket end, and rotationally from a release position to a locked position delimiting the movement of the locking sleeve from the second axial position to the first axial position. When the coupling is pressurized, a piston within the body is moved from a retracted position to an extended position delimiting the rotational movement of the locking sleeve from the locked position to the release position.