A valve assembly includes a housing having a valve seat defining at least part of a vent passage, a pressure responsive member carried by the housing and defining at least part of a pressure chamber, and a valve carried by the housing and having a closure moveable relative to the valve seat between an open position permitting fluid flow through the vent passage and a closed position at least substantially restricting fluid flow through the vent passage. The closure is moved by the pressure responsive member in response to changes in pressure in the pressure chamber. A control passage communicates at one end with a pressure source and at its other end with the pressure chamber and an electrically operated valve is moveable from a first position permitting a pressure signal to develop within the pressure chamber and a second position permitting pressure within the pressure chamber to vent.