I do not know when the A/C last worked on the car. There is a retrofit R134A connector screwed onto the low pressure side. As far as I can tell, no refrigerant in the system or super low.
Let me know if this is a reasonable approach to trying to evaluate the system's state of being.
Electrical things all seem to be in place, it think there's even a high pressure cut-out switch that is supposed to be used when going to R134A.
I am going to get a vacuum pump and do a long running vacuum pull and see if the system can hold vacuum at least for a couple of hours. If it can I will then attempt to put in some R134A through the low side, engine off. Once there's a can or so in there, I'll crank it up and see what if anything happens.
Longer term, I am thinking to disassemble and flush the system and replace as many o-rings as I can find. I want to convert back to R12 as I have a lot of it and I suspect it'll work better.
Does my evaluation approach seem reasonable?

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1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

No. If its flat there IS a leak. Fix it first use CO2 or nitrogen to look for the leak under pressure or you will introduce moisture into the system.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Hmmm, OK, I think I'll bring it in and have it leak tested first then. I've owned the car for 5 years and the A/C wasn't working when I got it. I will first try to fully determine if there's any pressure at all in the system. I think the hi-side may still have a valve I can reach to see if it hisses at all. This is my 5th 123 and I've never owned one with a working A/C yet. This one is going to be a retirement hobby to undergo a minor restoration.

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1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

You don't know what refrigerant was in it, and more importantly what oil it has in it currently. Hydrocarbon refrigerants like Duracool and Envirosafe use the same R-134A type fittings. I would use one of those since they work with any oil (mineral, PAG, ester) and work better than R-134A, even better than R-12. Better for the environment too. But read up and decide for yourself since many fear they are dangerous. I have used them for years and have no concerns.

Second that. Just fill it up with HC refrigerant and give it a go. It could be that the leak is so slow that it doesn't really matter.

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CENSORED due to not family friendly words