Abstract: A tank for volatile fluid is fitted with a diaphragm vent valve as an integral portion of a system in which both emission control regulations and safety regulations are met.Vapors are recovered in this system but more importantly, the tank such as the one on a tank truck, cannot be filled through a bottom loading valve normally provided near the bottom of the tank, unless the vent valve is open. The vent valve includes a disc which, when the vent valve is closed, sealingly engages the interior upper surface of the tank, and when opened, is pushed away from the interior upper surface. This design provides for a low exterior profile of the valve body, and a valve operable with relatively low air pressure.