Did you ever find a vacuam pump? If not, contact me again, and we will keep looking until we find one. I have a few other sources I can look up for you if need be.
Also, it seems that posting an R12 vs. R134 topic is getting like getting opinions on oil. All of the posts you've gotten are somewhat helpful, and are all with good intention. But not all of them come from Houston, Texas. There aren't too many large cities that strain an Air Conditioning system, both home and auto, like Houston.
The cooling capacity of R134 is less than that of R12. Also there are two types of oil to be used in an R134 system. PAG is one type and is strictly for systems which have never had R12 in them. Ester is another type and is the only one you can use in converted systems.
There are several downsides to converting, one of which is not a downside for people in climates where they don't deal with a Houston type summer, that downside is the reduced capacity. Theoretically, it is only about a 20% loss. But most a/c's are strained to the absolute edge of operation when in Houston. The 20% loss may be "the straw that breaks the camels back".
Also it seems that the R134 dries out the oil. Benzmac says it should be pumped down, oil changed and recharged every two years.
In spite of these downsides, I totally understand your dillema. R12 is RIDICULOUSLY expensive. I guess I shouldn't get started, but I think that when our govenment voted on banning R12, they should have all been required to live for a full summer in Houston before voting.
juan, I have converted a few vehicles to R134, one of them being a Benz, and I don't know what to tell you. I have been disappointed with the conversions, but losing $150 worth of R12 while troubleshooting a leak is a tough thing to do. It might be worth finding a GOOD independent auto air shop, handing it over to them and letting them find and repair the leak.
I think that this is probably a tough thing for you to do. It is very tough for me to do. I do all my own work, and not only is it difficult to cough up the money for this, but I'm almost admitting defeat when I do it. But I think that if you can find the right shop to find and fix the leak, you will be money ahead in the long run.
Best of luck and let me know if you still need a vac pump,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in