I finally got cold air in my '87 300D after a r134 conversion. After replacing the drier, expansion valve, rebuilding the compressor, replacing all the seals the sysyem still wasn't cold. The pressures were fine, aux fan worked and cycled properly. The fix in this case was to replace the serpentine belt. With a worn out belt, the compressor wasn't turning properly, thus low cooling from the a/c system. We replaced the shock and pulley while we were at it, now the car runs quietly and cools properly.
This might not be your solution, but it was something we overlooked that made the difference.

Hope you get it worked out

Steve