Mark wrote:
if it has a bladder and significant amounts of water come out the air
valve, that means the bladder has ruptured.... time for a new tank....
a very small amount of water is prob just condensation


and I always thought a bike tire and car tire used that same fitting so
I don't undertand what you are saying about that.

Mark, you are right. Thanks to your comment I double-checked... First,
I found this:

http://www.sheldonbrown.com/gloss_sa-o.html#schrader

Which confirmed that I actually have a pump for the Schrader type,
which is the standard automotive-style air valve.

That lead me to double-check why my first trial with the car tire gauge
did not succeed. It turns out that I was trying to be so cautious, that
I didn't press the gauge hard enough againt the valve.

So... I went ahead and used the gauge as recommended: The reading shows
something like 11.8 psi.

This is close enough to the optimal 12 psi (as specified by Amtrol), so
I must assume that the expansion tank's problem is a water-logging? The
stinky smell coming out of that valve suggests that indeed it may be
the result of water sitting there for a loooong time.

Comments? Suggestsion?

Thanks!
Sam