I don't know if I'm doing it wrong, but I had a couple tractors, a 946 Versatile and 8440 deere go down every year with the old R12.  I was going to change over to 134 but decided to try Freeze12, or whatever they call it. The 8440 I did 3 years ago and it will still freeze you out of the cab, and I put a can in the Versatile 2 years ago and it blew lots of cold this spring, and I've never had to add since I first put it in.