1980 450SL AC Problem

[toyfamily]

I just bought a 1980 450SL 2 weeks ago as a birthday present for my wife.
The AC worked during the test drive and for about 1 week later and then quit. Not knowing any better I was told by the previous owner to activiate the AC click the AC compressor switch on and push the DEF button in. This worked well to cool the car on hot days. Now that I know more about the car I determined that the Auto Climate Control System never did work--buttons don't do anything (maybe a later repair topic). Only the DEF button activated the blower and AC.

So my question is what should I look at to diagnose the AC to at least get it
working in DEF mode again? I will tackle the more complex Auto Climate Control System at a later date. Here is some helpful data:
- After starting car and turning on AC with DEF button, I get cold air for about the first 60 seconds and then it turns hot and stays hot
- The compressor turns on, runs, and cycles but I only get hot air.
- Yesterday I took it to a shop to check the freon level. They said it was low and added some along with some dye. No signs so far of leaks. That's all they could do since they were not familar with the Auto Climate Control II System and were honest and said that they were sure that I didn't want to pay them by the hour to learn it.

So for now I would just like to get the AC working again in DEF mode and then later when I have the time and patience I might tackle the Climate Control System.

Any suggestions on what to check? I would even consider suggestions on what to bypass, unplug, or jumper to get it to work temporarily.

Thanks,

Jim