Carl a couple of thoughts
1.) They are easily damaged (wiped out) by not releasing the tee screw on the back before turning.
2.) if the bronze core (in the rubber part) is corroded to badly there is a possibility of failure. (over pinking of the bronze)
3.) on the outside chance the rubber is torn or the housing has issues (like lighting strike going through it etc). they could also fail.

I am still using 3 of the original 6, and I have replaced 2 with the same type of valve (new at west marine outlet store back in 2001) they do not have anymore I am sure! I check back then. And glassed over the 6th hole.
SO if they are in good shape, they should be OK, but if not, you will probably need to switch valve type as supplies of these should all be gone by now. Fair winds: Norm

p.s silicon grease works great.