Mercedes Air Conditioning

My 1973 Mercedes 280C was purchased new in Texas, with – if my understanding is correct – dealer-installed AC. It has a huge York compressor. The system has been converted to 134, and holds a charge perfectly. It has been problem free until recently. When I start the car in the morning, the AC works perfectly. But if I go to lunch and leave the car parked, I get only a little cold air when I restart the car. In other words, I’m getting air that’s cooler than the outside ambient temperature, but not nearly as cool as it should be. The next morning, everything is fine again, until after lunch. The switch that controls the two fans works properly, and I just had the expansion valve replaced, which did not solve the problem. Can anyone offer a suggestion?

I would replace the dyer, which I don’t know why the person who did the work to begin with did not do that at the time of the expansion valve.