Finally found > 2,000 lb per year leak.
I had spent so much time trying to find where this thing was leaking. I used two tanks of compressed air one day last summer and search every visible inch but didn't find it. To make sure it didn't get too low when I was too busy to check and refill it, I rigged up an automatic fill valve that would put in the right amount when it leaked down. This thing had to be leaking over more than a ton per year. Finally I finally found a hole under the stairs and put a patch on it about 1700 today. I'll know tomorrow if that did the trick.