Abstract: A control valve for blocking atmospheric venting of gas fumes contained within a gasoline storage tank during tanker refill operations and which is adapted for attachment to the end of a venting tube coupled to the open space within the tank. The control valve comprises a tubular valve casing and a valve seat and seating member which are coupled within the valve casing, the seating member being reversibly displaceable along a flow channel therein between (i) a rest position toward the bottom of the casing which permits gradual air flow for pressure equalization of the tank and (ii) a sealed or seated position at the valve seat, wherein all flow of air or gas fumes is blocked. The seated position is developed in response to rapid exhaust of gas fumes through the flow channel, driving the seating member into the seated position. Loss of gas fumes into the atmosphere is therefore precluded during refill of the gas tank, requiring the tanker operator to withdraw pressurized gas fumes into the tanker shell.