[Amps] Vacuum Capacitors - refurbishing

[Eddie Hanham]

I just recently came by a number of vacuum capacitors similar to
UCSF-500.

Don't know much about them, suspect they are most probably war surplus!!

They have evidently received some water damage and I cant turn the
adjustment screw. 
The plunger (brass) is quite corroded - are they repairable?
Is there a spring holding them in or is it atmospheric pressure? 
How much force can I use to screw them ? Some plungers are out others
in!!

Can anyone advise if these are repairable and if so had any experience in
freeing them 
up.

Eddie

VK2BEH