1995 945, 8-valve, 225k miles

All, 'been trying to read up on the 940 AC system via the FAQ. The information is kinda... "all over the place" and I'm not sure just which problem I'm facing. Need some expert advice.

A little history...
Couple years ago, the AC in the 1995 940 stopped running. I realized that the securing screw on the clutch had fallen out and the clutch had fallen onto the engine protection pan. I retrieved it and reinstalled the clutch and installed a new securing screw. AC function was restored.

Last summer, the AC in the 940 stopped cooling. I found the pressure was low and boosted the system with a standard kit. This restored function.

This summer. AC doesn't work. Compressor is intact, coolant is fully charged, but nothing but hot air comes out. Midas checked out the car and said the compressor was bad and quoted $1,200" to fix it.

I don't believe them, but obviously the AC is not working. How can I diagnose this? Car is not worth $1200, but I don't have the dough for a replacement car right now.

I can replace the AC compressor myself, but I need to diagnose it properly.