Abstract: A gas pressure operated proportioning pump is provided including axially aligned and spaced opposed end pumping cylinders and an intermediate drive cylinder provided with centrally apertured opposite end cylinder heads. A piston is reciprocal in the drive cylinder and includes oppositely axially projecting diametrically reduced pumping pistons extending and reciprocal through the cylinder heads in sliding sealed engagement therewith and projecting into the adjacent ends of the pumping cylinders. The pumping cylinder adjacent ends include seal structure slidingly and sealingly engaged with the pumping pistons and the remote ends of the pumping cylinders include check valve equipped fluid inlet and outlet ports.