The original R4 compressors are notorious for siezing. Read about it both here and on GM web-sites. The one in my 85 300D seized and gave a plume of smoke under the hood as its clutch slid, melting the connector. Inspect your connector. Unsure about the one in my son's 84 300D. When he pulled in last summer after a 400 mi drive, the AC belt was broke and hanging down, and oil was dripping out everywhere. It had tangled in the pwr steering belt, flipping it over so its "teeth" cut into an oil cooler hose like a chainsaw, wearing a small hole in it. That sprayed oil everywhere, but fortunately still 2 qts left in the pan. I suspect his compressor seized then, though now I can turn it over by hand (I don't mean just the pulley).

Instead of R-12, I refill mine with Duracool, a HC mixture (mostly butane). It is compatible with all oils and works slightly better than R-12. I'll let you read the crazy rants from people who have never used it. You must do yourself since the EPA sets bizarre rules for shops, and most shun it for other reasons. However, it is quite easy. To draw a vacuum, I just connect to the manifold of one of my gas engines to draw down to ~20" Hg, then switch to a Mighty Vac hand pump using valves. I can pump it down to almost a pure vacuum (~29" Hg) in a few minutes. I let it sit for 30 min, pumping a bit to eliminate the out-gassing, then hold several hours, before adding refrigerant. If too weak to use a hand pump, you can buy an electric vacuum pump for ~$300, or use the vacuum pump already on your 300D.