I recently completely refurbished my AC system. I found 3 leaks -

Low pressure side schrader valve
expansion bock
Compressor shaft seal

I wound up getting a remanned compressor, new expansion block, new valves, new o-rings, and a new drier. Completely flushed the system with mineral spirits, and installed ester oil. Vacuumed the system for a solid hour, then installed R134.

In case anyone is worried about the efficiency of R134 in an E34 designed for R12, I am currently seeing temps as low as 35F in 90F ambient temps.