I have to admit, I have always been fearful of AC systems such that I never tried working on them. Funny too because most of the old cars I owned had issues with failed AC systems. But after 3 AC charges in as many years, I decided to overhaul my system and get is working right. The main issue in AC systems is that seals wear out, especially after converting from R12 to 134a refrigerant.
The main thing you need to do is replace all the seals in all the components. There are some wear parts like the expansion valves (2 if you have the auxiliary AC) and receiver/dryer. You also need a bottle of refrigerant oil to help with assembling all the parts.
Replacing the seals in the piping is as simple as plumbing, unscrew, replace seal, re-assemble. You need to get the correct seal kit and thatís were specialized 928 vendors are really helpful.
The AC compressor usually fails because the main shaft seal fails or it runs low on oil. The seal kit is about $40. A good quality rebuild compressor is $400-$500. A new compressor is $100+. So it definitively make sense to experiment for $40. Fortunately there are a ton of good resources on the web to help you. I found schematics, videos, writes all very useful. You can find the compressor part number on the unit and do some searches. Here were a couple very useful schematics

Here are some pictures of the disassembly process. I was amazed had how well the compressor looked inside. It looked new and I was equally impressed with the design.

After I had apart I realized it was not so big and scary after all. I did make some mistakes re-assembling the unit and had to order a second seal kit. But this was still an excellent learning experience! Everything is put back together now and I am waiting for warm day to take it in and have it tested and recharged. Fingers crossed.
UPDATE: Summer came and went I am happy to say I have nice cool air in my 928 again. The compressor rebuild worked great. When I took it in for recharge, they pressure tested the system for leaks and none were found. I had them put UV dye in the system and I purchased a UV flashlight so I can check for leaks myself. No leaks found yet. I have to say this AC business is not as complex as I thought. Itís just tinker-toys and rubber seals.

Benno 928 (thanks for sharing...)