Abstract: A pump assembly especially adapted for pumping viscous and/or abrasive materials is comprised of a pair of opposed single acting piston pumps operated alternately by an interposed reciprocal actuator, and a static chamber interposed between each pump and the actuator; each pump comprising a pumping chamber having aligned inlet and outlet check valves in its peripheral wall and defining a straight line path of fluid flow diametrically through the chamber, a piston having its periphery spaced from the peripheral wall of the pumping chamber and having a relatively short stroke to maintain the straight line path of fluid flow through the chamber, and an annular seal extending from the peripheral wall of the pumping chamber toward the piston and bridging the gap between the periphery of the piston and the peripheral wall of the pumping chamber, the piston having sealed engagement with the seal throughout its reciprocal path of movement; the static chambers isolating the two pumps and the actuator from one another