whereby said front and rear face portions form hydrodynamic pressure fields before said medial narrow gaps at forward movements of {TAIL ~ said piston shoes} and hydrodynamic pressure fields rearwards of said narrow gaps at rearwards directed {HEAD ~ movements} of said piston shoes, and 
whereby said front and rear face portions form hydrodynamic pressure fields before said medial narrow gaps at forward movements of { TAIL ~ said piston shoes } and hydrodynamic pressure fields rearwards of said narrow gaps at rearwards directed { HEAD ~ movements } of said piston shoes , and
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 OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH