Abstract: A main valve and separate sensing means are mounted in a housing spaced-apart relation. The main valve is formed with a valve chamber having an inlet port and an outlet port, and the valve comprises a valve seat and a valve member cooperating therewith. The valve member is movable between a closed position in which it engages the seat and blocks flow of fluid from the inlet to the outlet and an open position in which it is spaced apart from the seat and enables such flow. The sensing means includes an exhaust to the atmosphere and exhaust shutoff means actuable for closing the exhaust. The housing and main valve are formed with a passage enabling restricted leakage of fluid from the main chamber to the sensing means for exhaust to the atmosphere when the valve member is in the closed position. The leakage fluid is in communication with the valve member and when pressurized applies a force urging the valve member to the open position.