A gas powered pump tank is alternately filled with liquid such as water at low pressure, and then emptied by gas pressure such as from pressurized steam. An auxiliary exhaust assist valve opens at a threshold pressure after switching from the pumping phase to the filling phase to more quickly vent the pressure in the tank and minimize obstruction of the incoming flow of liquid. The pump tank has inlet and outlet check valves for the liquid, permitting flow only in a positive pumping direction. A gas inlet valve and a primary gas exhaust valve are controlled by an alternating mechanism such as a float with a snap action toggle that opens one of the gas valves while closing the other. The auxiliary exhaust assist valve has a valve body spring biased inwardly of the tank, and opens at a pressure lower than the pumping pressure, thereby venting the tank more rapidly and also assisting in venting any additional steam produced by boiling of the water or other liquid. The auxiliary exhaust valve can vent into the gas exhaust port, being associated with the primary exhaust valve, or into the open. The auxiliary exhaust valve can be partly controlled, and in particular held closed, by the float toggling mechanism, especially when arranged to vent at a relatively high pressure in proportion to the pumping pressure.