Abstract: In a mobile liquid supply tank, such as a water tank for use as fire fighting apparatus, a hollow tank is provided with at least one fill neck and an overflow relief valve on the upper surface portion of the tank. A pressure fill valve is also provided near the lower portion of the tank for emptying or filling the tank. Each fill neck includes a buoyant float disposed within the tank that seals the fill neck when the tank is substantially filled with liquid. The overflow valve, adapted to open when the pressure within the tank exceeds a predetermined value, is preferably located in a recessed portion of the tank. A drain pipe extends from that recessed portion through the tank and exits from the bottom of the tank to discharge liquid overflow away from the forward path of the rear traction wheels of the transport vehicle.