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