Abstract: A piston pumping system is disclosed having a pair of piston pumps operated by plungers with roller follows which ride in rotary cam plate grooves. The grooves are arranged such that the delivery of each pump overlaps the suction of the other pump. Means are provided to sense any pulsations in pressure to change the cam speed during the precompression phase of operation and means are provided to sense delivery pressure and to adjust cam plate speed during the interval between the initial phase of delivery and the end of the precompression stroke.