A hydraulic pump/motor (110) having: a hydraulic chamber (112); a primary piston (114) for reciprocation in the hydraulic chamber and mechanically coupled to a connect rod; a slave piston (116) for reciprocation in the hydraulic chamber; a inlet/outlet port (119) for admitting and expelling hydraulic fluid to and from the hydraulic chamber; and a resilient mechanism (118) for, in conjunction with the slave piston, compensating for a difference between hydraulic fluid in the hydraulic chamber and a volume of the hydraulic chamber; wherein the primary piston and the slave piston reciprocate responsive to movement of the connect rod and to injection and discharge of the hydraulic fluid to and from the hydraulic chamber.