a valve seal disposed on said plunger in {HEAD ~ the vicinity} of {TAIL ~ said downstream end} thereof; and 
of
a valve seal disposed on said plunger in { HEAD ~ the vicinity } of { TAIL ~ said downstream end } thereof ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH