I know what you are going through. My 86 SDL (209,000 miles) and I have had so many A/C problems that it is not funny any more. My last one was solved as follows:
Sitting on top of the compressor, there are a series of electrical connections. These are hard to see and they collect lots of grime. After checking many of the usual problems, (Fuses, Relays etc.) we cleaned these with some simple green and now (for now) all is working. There are so many causes for the A/C not to work, MB should be spanked for this system. Also be sure to look at No.5 fuse and change to at least the blue fuse as well. Hope this helps.
Ron