Maybe I can tag along on this thread.
I had a bad AC experience when my '79 300TD was converted to 134A. Before that it blew cold. I had the shop replace the compressor with a new one I was lucky to find with proper fittings. Kept the series flow condenser, but now have a new parallel flow unit.
Following the "upgrade" to R134a cooling diminished over a few months but since we don't have hot weather all year round it was a year later before I turned it on and - nada!
I think many AC shops get in a hurry they don't flush the system completely and they fail to evacuate long enough.
That plus using dye for leak tracer when a sniffer behind the dash is in order to detect leaks in the exp valve.
My friend used to do this work - his approach to AC was like a scientist but he moved away after he retired now I have no expert free advise and wrenching help so I am looking for a good AC shop in the SF Bay Area.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!