[Milsurplus] How Much Vacuum in "Vacuum Tubes?"

[David Stinson]

"Vacuum" is a relative term.
Just how much "vacuum" does one need to make a vacuum tube work?
Our valves can't be at perfect vacuum; no way that thin glass
would hold up.  So is it possible for an amatuer to create a 
vacuum pump and chamber good enough for tubes?
Kinda thinking about how to recover rare or expensive
tubes that have gone gassy or low-em.
I have engraved-base, big-globe types 80 and 50 
that would look soooo good in a classic '20s homebrew,
if only.....