I had the dreaded leaky a/c system on my 1987 300TDT when purchased 1.5 years ago. It would leak down from completely full (and frosty cold!) to worthless in 7-8 weeks. It was leaking roughly 1lb/month of refrigerant.
I purchased a decent used sniffer off ebay. Had small leaks in the condenser and expansion valve. Didn't verify it, but strongly suspect the majority of the leakage was in the evaporator.
So I was looking at a repair cost on the same order as the Donald's.
I decided to try out the dreaded cryo-chem. I figured it'd be an interesting experiment. If it trashes out the system, I'd just send the car in and have everything replaced - got the money in the bank
Here's what I did. Purchased the cryo-chem kit, a new dryer, refrigerant oil, and R-12 (I'm 609 certified). Rented a vacuum pump from my local rent-all place.
Had the remaining freon recovered at my local service shop, then opened the system, added oil, and replaced the dryer. Pulled a vacuum for one hour, then followed the cryo-chem instructions for filling the system.
I did this work approximately 1 November 2000. The a/c worked fine all winter and spring. I checked it when road trip season approached, about 1 April 2001. It was down just a bit, so I added about 10oz R-12 to top up the system.
Since that time it appears to not be leaking at all - no bubbles in the sight glass, a/c outperforming MBs specifications, etc.
I'm not quite ready to call it a success yet - it hasn't even been a year. I'm still waiting for the cryo-chem sealant to clog the system, or some such terrible thing. However, it hasn't happened yet.
Here's the interesting part. My a/c works as well as Donald's. (Better, actually - I'm still runinng R-12.) I spent $275 to repair it, and about three hours of my own labor on a Saturday.
I've found a thread or two on cryo-chem here in Mercedesshop. One of the experts has used it - I *think* it was Benzmac, but don't quote me on that. Success was mixed - sometimes worked, sometimes not. So far, it looks pretty good on my 124 wagon. We'll see how it performs over time.