Denny,
I have no idea how to test such a thing. It's a sealed unit with no signs of deterioration so I'm going on faith that it worked before the seal went south.
With your shop and dedication to overcoming the smallest problem with patience and time to invest this project would be right up your alley. Good cores come up occasionally on eBay so you shouldn't have that much difficulty getting one. You could work out the kinks on your unit and then either simply replace your tube abd bellows with a good one from a donor or start over with a fresh core.
Funny, while I was doing this two thoughts kept running through my mind. One was that it would be a perfect project for Denny Graham and the other was that Andy Griffith episode... "Aunt Bea, call the man".
