This thread got me interested in looking up some references on sodium-filled valves and I found this site:
http://www.peebles34.freeserve.co.uk/Pages/Big%204/THE%20BIG%204.htm#va
I've never seen it before, but it's got a lot of great info on T4 applications. Crummy page design, though - make sure you click on the "i"'s to see hidden info.
Regardless, it has some of the best explanations and pictures of valve problems that I've seen anywhere.