A closed circuit power system is disclosed herein having a hydraulic power device incorporated therein which includes an input piston and cylinder assemblage and an output piston and cylinder assemblage connected together by a fluid conduit so as to operate in unison. The input assemblage includes a piston moved by an input ram so as to apply a fluid pressure via the conduit to the output assemblage. The output assemblage includes a pair of pistons operating in unison in separate chambers in response to the applied fluid pressure for moving an output shaft. The pair of pistons include pressure faces against which the fluid pressure is applied which are of equal area but different shape. A selected piston of the output assemblage includes conical forward and rearward faces with a corresponding or matching concave recess in the rear cylinder wall.