Preferably the bias means is a spring, {TAIL ~ e.g. a helical spring}, retained over {HEAD ~ the stem} of the valve member 
Preferably the bias means is a spring , { TAIL ~ e.g. a helical spring } , retained over { HEAD ~ the stem } of the valve member
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH