The body has a fluid inlet port for receiving pressurized fluid and a fluid outlet port connected to the retention balloon. A first passage connects the fluid inlet port and the fluid outlet port. A second passage in the body is connected to the balloon fluid return path and is at the pressure of the retention balloon. A valve prevents fluid flow through the first passage when actuated. The valve includes a pressure-responsive member movable to a position to obstruct fluid flow in response to fluid pressure in the second passage exceeding the predetermined level. Flexible means such as a membrane defines a normally open portion of the first fluid passage, which is closed by the moveable means bearing on the membrane when pressure exceeding the predetermined level actuates the valve.