and additional guiding means including tongue and slot guiding elements connected between the respective carriages and said port member for guidingly confining the carriages in a predetermined spaced relationship relative to {HEAD ~ the port member} for maintaining predetermined compression of said valve members and thereby maintaining sealing pressure between {TAIL ~ said valve members} and said port member, 
and additional guiding means including tongue and slot guiding elements connected between the respective carriages and said port member for guidingly confining the carriages in a predetermined spaced relationship relative to { HEAD ~ the port member } for maintaining predetermined compression of said valve members and thereby maintaining sealing pressure between { TAIL ~ said valve members } and said port member ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD 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