I had the same thing happen to my 1980 300D about 6 years ago. I had the time to put the car out of commission for a while and did so. Decided to get the M/B A/C systems manual and got to chasing down the problem. Once I had the dash and center system out, I decided to take it a few steps further. Knowing that North Texas is no place to be without a reliable climate control system in the car, I went ahead and replaced all the vacuum actuators and switches. It wasn’t that hard of a job, nor very expensive. Remember also, that you can replace just the diaphragms on the vent actuators without having to get the whole thing. Knowing I was running the original servo, I replaced it also. I still had the problem you have when I was done with all that.
Here’s one place you might not think to look. But, where I found my problem to be. Pull the battery and split the case on the bundle of vacuum lines running into (and out of) the car from the servo. Seems battery fumes can get into that casing and rot the plastic vacuum lines. I had two of the lines just crumble as I pulled the sheath back from them right by where the battery sits. The casing/sheath looked fine. You have to look DEEP inside. Clear into the area behind the glove box. I tried to repair the lines by cutting and splicing with tubing. That showed for sure where the problem was, as the system worked fine after that. At that point though, I decided to just buy a new vacuum harness and install it prior to putting the whole thing back together again. Someone had told me that replacing much more than an inch of the hard vacuum tubing with rubber hose was not a good idea. So in went a new vacuum harness. About 125.00, I think it was.
I also converted to 134A at that time.
Another thing to keep in mind is the quality of your coolant, and the fact that it runs through the bottom valves on the servo. Any crap in the coolant seems to jam things up in the servo, and that’s not a good thing at all. Jam the bottom valve and you stop the vacuum valving that goes on up top.
I change and flush my cooling system every year. I pull the lines off the bottom of the servo and flush it out real good too. Once a month, no matter what time of year it is, I run the system at max heat as well as max cool for a few minutes. I’ve not had an A/C issue since.