Before I did anything major to try to fix a band organ suffering from low
vacuum, when the organ played fine before being put away for the winter,
I'd just try running it continuously for a few days, giving it a chance
to settle itself.  After a long period of disuse an organ needs a little
time to get the stiffness out of its bones.  Chances are this won't solve
a vacuum-loss problem as severe as Gordon describes, but it might, and it
won't cost anything.