Thanks all. As far as knowing it's weak, I did manage to charge the system, and the pressure never got more than 200psi or so, even after running a while. The system held freon for a while, but would not hold a vacuum for more than a few minutes.
Yesterday, I swapped out the compressor with a new Nippondenzo compressor. Those allen head bolts are a real pain to get to, but managed. PB lubricant made easy work of one, but the other refused - spent about 2 hours trying, and ended up using a dremmel to grind the head off. The stud then easily came out, and I replaced with new.
Now the system holds a vacuum! so it seems it was the compressor afterall. There was basically no oil in the pump. The stuff that dripped out was the UV die. Now I know where it went. Anyway, car is back together, waiting to be charged this weekend. I may give the nitrogen a shot just to help remove moisture - thanks for the info.

Only thing I have to figure out is what the schrader valve on the compressor manifold is for, and do I need to do anything with it. I replaced the stem while I had the compressor off, but does the system get vacuum'd there, or filled?