my AC hasn't been working. it seems like the expansion valve is stuck/clogged. i've got a new drier and expansion valve coming from FCP groton. i was looking at possibly using this ES-12a stuff [http://www.btt.7p.com/AR/FAQ.htm] as the refrigerant instead of R12. has anyone tried this stuff or think it would be a big mistake and just shell the money out for the R12?

You'll get all manner of responses, opinions, and commentaries on this matter. First make sure it's actually legal to use it, many hydrocarbon-based refrigerants aren't actually permitted for use in cars. It almost certainly won't last as long as R12 in a well-sealed system because relatively speaking, R12 is made up of big molecules. Same reason R134a bleeds out faster. And many people report failed compressors following refrigerant changes, but I suspect that's got more to do with putting a component back in service after it's been sitting idle for months or years at a time - a dried-out compressor seal that hasn't held pressure for a while is bound to fail shortly after being put back in use (hence the importance of cycling the a/c once in a while even in winter). And it's not as simple as topping up the R12 with substitute - the system must be evacuated completely first. If you know you've got a tight system that's holding pressure and you've been using it until recently I'd probably stick with R12, it's expensive but it works well. This could be the time to look at R134a, too, but if you'll just need a little R12 to supplement the existing charge that'll be recovered from the system, stay R12. FWIW I am considering an R12 substitute for my 740, only because it's currently cheaper than R134a here, and the system has been opened and not used for a while, I expect something to fail anyhow when I charge & use it.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina

Here's one of those opinions:

I have used a product called Enviro-safe (ES-12a) in a few vehicles, including our 1991 Volvo 240. These hydrocarbon blends have to be used in a leak-free system. I used it in an old Mercedes, and it was excellent. I got 38-degree air from the dash! I used it in my parents' Vanagon camper, and it worked great for a little while. It had a leak, and performance degraded rapidly. I put it in the '91 240, and it worked extremely well, as long as the car was moving. The problem with the '91 (not earlier 240's) is the condenser fan control. It works off of a pressure switch in the liquid line. When the refrigerant gets too hot, indicating that not enough heat is being removed by the condenser, the pressure goes up and turns on the fan. The Enviro-safe operates an a much lower pressure than R-12, and won't operate the switch. I took it out (the refrigerant) and recharged it with R-12. My neighbor had a stash of it in his garage, and sold me some cheap. I don't know what the '88 760 uses for condenser fan control. If it uses the pressure switch, maybe you could bypass it. If I couldn't get R-12 cheap (I can't anymore), I would hook the fan up to the compressor circuit with a relay. Enviro-safe is available from autorefrigerants.com, and they have a discussion regarding the legality of it.