Listen to Benzmac and buy a NEW compressor. I went through the same thing you're going through and really made a big mistake with a rebuilt R4. It was before I found mercedesshop.
To properly flush, you will need to break every line and flush the daylights out of everything you can get near. You can get a flush unit which is a can with a hose and blowgun, it also has a schrader valve to pressurize it. You pour in the flush, air it up at the schrader valve and blow. The flush that I used was alcohol with some sort of citrus.
Another very good thing to do, and I wish I had known this when I went through this problem, is to put in a suction side filter. You still need to flushe the system, however. You can get them at the a/c supply house. Put it in the softline just before entry to the compressor. Had I done that, my wife may not have gotten rid of her beautiful 300TD.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in