Abstract: A pressurized air pumping apparatus for pumping for example drilling mud and like viscous or slurry materials comprises a fluid receiving sump tank having an upper inflow opening and a valve seat at the opening. A discharge conduit at the lowermost portion of the sump tank discharges fluid to be pumped therefrom. A source of compressed air is provided by pressurizing the sump tank and a vertical conduit enters the tank and introduces pressurized air into the interior of the sump tank with the vertical conduit providing a discharge port for entering the compressed air into the sump tank. A valving member is movably mounted in preferably a sliding fashion within the sump tank upon the vertical conduit with the valving member sealing the conduit discharge port in the normal inoperative state while sump tank is receiving by gravity flow for example inflowing fluids to be pumped.