A diaphragm operated pressure responsive valve has its inlet connected to a fuel tank vapor line and the valving chamber openly connected to the storage canister and to the engine air intake manifold vapor purge control valve. The diaphragm is operable to vary the spring bias on a valve member disposed in the chamber for biasing closed the vapor line from the tank in the event of high vacuum from the engine purge control valves. A bleed bypass permits continued bleed flow of vapor from the tank to the valving chamber.