[Speedslug]

In my opinion the sieve beds shouldn't have any problem being 'stored' in humid area, it's being 'used' in high humidity that could cause problems.

As for storage, I understand that there are tiny spiders that like to build nests in the small holes like the outlet ports of machines like oxycons.

The other thing to look for, if you take off the cover, is cracks in the tubing one the inside.
These machines are designed to get overhauled and recertified for medical use every 2 or 3 years and they often used to have the tubing replaced along with the filters as a routine without even checking for problems.

The place I would look would be the filters.
You will have to see if you can get a filter kit for your specific machine, I think Salt City Glass out of Utah used to have kits for sale at one time.

There were often two filters on the intake side and a third one behind the control panel just before the output connector.
That third one was a bio filter that prevented bacteria from being passed to the lungs of the patient.
Being that it had really tiny micro pores it could have gotten clogged up in a humid environment.

It wouldn't hurt to remove the filters for a test run of a half hour to see if that fixed it but I wouldn't run it without filters for any longer than that.

You really do not want dust to get into the sieve beds as that will shorten their life for certain.

Look for cracks at the fittings of the tubing.
The tubing can be replaced with stuff from the local hardware store.

Good luck.

Oh and if you are going to get one from Craigs List, you can remind the sellers that selling one for medical use without a prescription is against the law and constitutes practicing medicine without a license so grandpas old unit is ONLY good for welding and jewelry torch work.