I don't recommend this method. Sorry, but you're asking for trouble if you don't know if it's got R12 or 134 in the system or not.I'm not sure I understand your reasoning. I agree that safety should be first priority. However, it would seem to me that being able to drive the car no matter what the weather would be my second priority. Very often, the issues with old A/C systems can be narrowed down to leaking o'rings or dried seals.
I bought a car once that the owner said had a bad A/C system. He discounted the car an additional $400 because of it. When I got to diagnosing the A/C issues (I live in Louisiana and it gets hot down here) I found the problem to be a loose schrader valve. I tightened the schrader, pressured it up with air to find that it didn't leak. Blew it down, replaced the accumulator/drier and Pag oil in the compressor, pulled a vacuum, charged it up and had A/C the entire time I drove the car. In three years I put additional gas in it one time.
The point I'm trying to make is that it won't cost you any money to pressure the system up with air to see if, and how bad it leaks. I'm pretty sure you could even see if the compressor will come on using the air pressure. Don't let it run any longer than it takes to see the compressor engage & turn. If it engages and turns, a new accumulator/dryer, new Pag oil, 134a o-rings, long system evac and some 134a could put you in air conditioning.
This is not how it should be done but it is probably the least expensive way to diagnose what you have. Since the system is has been empty for 26 years, you really don't have anything to lose.
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