Hmmmm, this sight-glass thing seems to be misleading, at least to me. When my '91 was brand new, and cooling like crazy, the glass had a foamy, moving liquid look to it. No big bubbles, you could just see the liquid moving by in sort of a foamy-looking state, at any RPM.
After 12 years, it started to not cool so well, but the sight glass had that exact same look - sorta foamy liquid going by.
I did a pressure check, found it a bit low, and added one can of R-12 from my secret cache. The pressures came right up and it cools great again - but the glass still has that old familiar foamy look.
So I learned nothing from the sight glass. But I must admit that one can of freon in 12 years ain't too shabby.