I can not diagnosis your problem but I can suggest you post your questions on the A/C website whose link I have below. A few weeks ago I replaced compressor, expansion valve and reciever/dryer on my 92 300E and they were extremely helpful.

As far as getting your system filled again you will need to evacuate the system with a vaccum pump to remove moisture. Since you may need to take something apart again I would not immediatley put in a new receiver/dryer. You will need a set of hoses with manifold gauges and I think 1.6 lbs of R 134a to recharge. Read information on the site I gave the link to and it will explain how to do this. It's not hard to evacuate and charge system it's hard to diagnosis the problem as you have already experienced.

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