A fluid pressure operated piston engine assembly, such as a pump assembly, including a fluid pressure operated piston engine and a fluid valve for coupling fluid under pressure to alternative portions of the piston chamber of the piston engine. As disclosed, as the drive shaft of the piston engine approaches each end of its stroke, fluid under pressure is coupled by the fluid valve to a portion of the piston chamber to effect reversal of direction of travel of the drive shaft. The fluid valve includes a valve rod which is translated to effect the alternative modes of coupling pressurized fluid to the piston chamber. To accomplish this a drive frame is mounted for limited translation on the valve rod and driven by a pair of pins, attached to the piston engine drive shaft, at the ends of the drive shaft stroke. The drive frame carries a pair of flat springs having contoured portions bearing against a drive block attached to the valve rod, and as the drive shaft approaches each end of its stroke the pins engage the drive frame so that the springs co-act with the drive block to "snap" the valve rod to the alternative valve position.