Probably this posting come as a bit late as the nipple had already broken. Had similar experience when first time undoing the big nuts at compressor ports. Used WD40 then continue to untighten slowly bit by bit, then another spray until they're free. A bit of WD40 gets into AC line doesn't matter much. Nowadays I wet the threads with ATF before assembly (careful not to get into the ports). I have no more problem in undoing those big nuts even when attempting repairs many years later. Did the same with the threads at orifice tube assembly. No more sticking.

Amarin.