I swithched my 1989 260E to R-134 about 5 years ago. I did not change anything, except for the fittings on the hoses. Same condensor, compressor and everthing else.
After reading this post, I went to check air temp. at the dash vent.
No refill in the last 5 years, outside temperature, currently 88deg. and very humid Air temp. at the vents : 41deg.F.
I think you are looking at problems taht do not exist.