If a scuba tank is "too old" for underwater service (and I'm not sure what
that means exactly) -- I am going to *assume* it means that it would also
fail a hydro test...if that is the case, it is unsafe and illegal to fill
such an old tank...so you're looking at one shot with how ever much air
is in the darn thing and then toss it.
I respectfully must advise against the whole idea.