I can tell you that a Grand Cherokee is almost as difficult as the 124 evap.
After maybe a thousand conversions, probably at least a hundred on 124 cars, I don't have the momentary luxury of glorious results. I'm happy that John's system is working well, but from vast experience I can say that yes, the reason the new system appears good is the poor performance of the old system - not the 134.
I would also say that ester oil is not the way to go. I agonized over this for a long time as ester was the original mechanism for conversion, the "Holy Grail" so to speak. My worries came because as we started using only MB rebuilt compressors we became worried that failures wouldn't be covered under parts warrantee if the proper procedures weren't followed. MB often does failure analysis and kicks back stuff they won't warrantee. (warrantee is not a gimmick at the wholesale level).
What I found over time and investigation is that the viscoscity of standard ester oil was 100. PAG oils come in 46, 100, and 150. The Nippondenso compressors used on MBs use PAG 46. Experience has shown better cooling and longer lives of compressors with the lower viscoscity oil. My suspicions are that many of the brutal failures of the early 124 conversions were the viscoscity and over oiling.
Bosch Master, ASE Master, L1
33 years MB technician