Abstract: A mobile tank for transporting liquids has a vent valve with a safety device to assure that the vent valve is always open during draining, to relieve vacuum build-up. The vent valve is coupled to a vent port in the top of the tank, and operates on air pressure. The air pressure is controlled by a control valve mounted near the discharge valve. The control arm on the control valve is mounted in an interfering position with the discharge valve, requiring the air control valve to be opened before the tank discharge valve can be opened.