The position of the vent can be moved to {HEAD ~ either side} of {TAIL ~ the piston} 
of
The position of the vent can be moved to { HEAD ~ either side } of { TAIL ~ the piston }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL