I recently rebuilt my AC and had a fight with that one and the fitting about that size coming off the evaporator. In both cases I used two wrenches with a piece of pipe over them so I had an extra 3 feet of leverage. Oiled the threads with PB blaster or whatever I had around and went to town, they broke loose wit quite the bang.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...