When you pulled a vacuum on the system did you leave the guages on and with the vacuum pump off to see if the vacuum in the system bled off, showing a leak in the system? All the dyes that I'v used were put in when the system was under pressure so you could trace where it came out to find a leak, putting it in under vacuum will just keep it in the system. In any particular system R12 will give you more cooling than 134 but you need to be certified to work with or buy R12. Also R12 is expensive. I would change all seals and the compressor and fill the system with 134. 134 and the charging equipment is available at any auto parts store unlike R12.

__________________
1983 300SD
200000miles