An assembly with a piston reciprocated with the aid of a floating head in fluid communication with the piston. The assembly may utilize a floating head that is shifted in position to promote reciprocation of the piston through the aid of pressure supplied to the floating head from a pressure volume regulator. Alternatively, the floating head may be in fluid communication with the piston at one side of the head and isolated at the other side. In this manner changing volume and pressure at this other side of the head during reciprocation may ultimately lead to floating head movement toward the piston, thereby promoting the continued reciprocation. Additional efficiencies may also be realized through unique hydraulic layouts for both operating and working fluid circulations.