Abstract: A pump/motor includes a back plate having first and second fluid ports configured to be differentially pressurized, first and second reaction plates rigidly coupled to the back plate, and a valve plate slideably coupled to the back plate and having first and second fluid feed channels configured to receive fluid from the first and second fluid ports. A plurality of hold-down pistons is positioned in respective hold-down cylinders formed in the valve plate. Each of the hold-down pistons is configured to be biased, by pressurized fluid in the respective hold-down cylinder, against a surface of one of the reaction plates. A barrel, having a plurality of drive cylinders, is rotatably coupled to the valve plate. Drive pistons positioned in the drive cylinders are biased against a thrust plate by pressurized fluid in the drive cylinders. The thrust plate is coupled to an output shaft of the pump/motor.