Its too bad you had such trouble with an R4. I probably had its brother last week but I've been there before after flushing ten times we were still getting black flush solvent. Too bad cause that relatively nice 83 300SD is probably history after we gave her a price for evaporator and condenser replacement. We'll see when it gets really hot how she handles it.
After over twenty years of replacing 50 units or more a year, I can speak with experience that the R4 is as good as it gets. Problems come (from rebuilts always) from incomplete repairs. If you want a really junk compressor you can look at all the Nippondenso compressors used since 1986. They have teflon coated pistons and fail in a condition know widely from Fords licenced Nippondenso copy "Black Death".
They almost always fail like that rare R4 failure. Their problem is that the burned up mess of Aluminum. teflon, oil and refrigerant forms a temp dependent liquid that hardens in the condenser and when flushing it looks clear but upon reuse the mess turn liquid again when the condenser heats up. It gets worse on the 300e because the evaporator is a multiple entree unit and when flushing it is real easy to leave flush in the evaporator. As the gas pressure clears out the upper level and all the flush force then goes through the clear tubes and the lower tubes retain flush wasting the new compressor.
Nem R4 units (with clutches) can be purchased from your AC-Delco jobber for a bout the same price as a Sanden, whuch is as cheap as it gets. You can probably kill a good system with an Autobone rebuild for less than 70 bucks.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician