Abstract: A dual tank storage system for fuel having gaseous and liquid phases where one tank is a main storage tank and the other is an expansion tank. The system has a valve assembly on the exterior of the system for permitting fuel to flow from the main storage tank to the expansion tank in order to relieve pressure in the main storage tank. The valve assembly permits only the main storage tank to be filled. The fact that fuel can flow from the main storage compartment to the expansion compartment to relieve pressure from the main storage compartment allows the main storage tank to be filled to 100% capacity. The valve assembly includes a valve chamber and a valve plunger movable in the chamber between a normal operating position and a tank filling position. A coil spring biases the plunger towards the normal operating position. A filling port for fuel leads to the valve chamber which opens into a port of the storage tank. Fuel is drawn only from the expansion tank.