whereby {HEAD ~ each} of said front and rear face portions draws an exterior fluid into said front gap portions of said front and rear face portions at forwardly directed movements of said piston shoes and into said rear gap portions of said front and rear face portions at {TAIL ~ rearwardly directed movements} of said piston shoes, 
whereby { HEAD ~ each } of said front and rear face portions draws an exterior fluid into said front gap portions of said front and rear face portions at forwardly directed movements of said piston shoes and into said rear gap portions of said front and rear face portions at { TAIL ~ rearwardly directed movements } of said piston shoes ,
OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH