They don't normally wear out but they can fail. If the valve doesn't close when inverted it is no longer any good. Plus 
with age they become brittle as you discovered. I have seen
some people repair the valve my embedding a brass barb into some epoxy on top. YMMV as always...

Bill

On Jun 29, 2012, at 1:00 PM, "Stuart MacMillan" <stuartmacm@gmail.com> wrote:

> Found an '82 in a junk yard, I'm on my way to pull the expansion tank soon.
> Not paying more than $10. I checked the GoWesty site where they are $40
> including shipping.  I'm glad they have so many parts, but this one doesn't
> wear out, so the used option ....