Abstract: Improved mud pump comprised of plural cylinder pumping units, each cylinder unit consisting of a pumping compression chamber and two or more hydraulic driven expansion chambers, with one or more expansion chambers being employed to drive a pumping plunger to cause fluid to be pumped through the compression chamber, also one or more of said expansion chambers being employed to return the plunger; the plunger further being employed as the housing for one or more of the said expansion chambers. The invention includes novel hydraulic drive circuitry and novel hydraulic distribution valving means for control of the hydraulic drive fluid that is utilized to drive the expansion chamber of each cylinder pumping unit. The invention also includes novel positioning means for the unidirectional inlet and outlet mud pumping valves.