Just a quick note about the VDH clear silicone mushroom replacement valve.
I have been messing around with my 1980 Conshelf XIV for a while and never been quite happy with the way it breathes, in particular the "magnum" exhaust seemed a little restrictive although the yellow mushroom valve looked to be in perfect condition. I finally summoned the motivation to change the valve for a VDH unit that I had for a while and WOW- what a difference! Seriously it is night and day! It now feels like any modern second stage to exhale through and it seems to improve the overall breathing experience markedly. If anyone is still using the factory yellow valves I can heartily recommend the VDH replacement as a worthwhile and cheap Conshelf performance upgrade.