Abstract: A radial piston pump wherein the crank pin of a crankshaft engages with arcuate shoes at inner ends of radially extending pistons which are reciprocable in discrete cylinders. The outer end of each cylinder normally bears against the convex sealing surface of a discrete back support having in the sealing surface a pressure port for evacuation of pressurized fluid in response to outward movement of the respective piston. The cylinders are caused to move their outer ends away from full engagement with the respective sealing surfaces in response to inward movements of the respective pistons so that the chambers of the cylinders can draw fluid from the interior of the pump housing. The cylinders are caused to move substantially radially and away from the respective sealing surfaces due to frictional engagement between their internal surfaces and the external surfaces of the respective pistons.