whereby 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 {HEAD ~ said rear gap portions} of {TAIL ~ said front and rear face portions} at rearwardly directed movements of said piston shoes, 
whereby 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 { HEAD ~ said rear gap portions } of { TAIL ~ said front and rear face portions } at rearwardly directed movements of said piston shoes ,
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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH