i had the same problem with my monovalve and had to bang it out with a screwdriver and hammer from the other side. i would not pry it out as you could damage the washers which you will need if/when you replace it. keep in mind that i only treated the monovalve that way because i already had a replacement kit ready to go. don't bang it out without one as it will mess it up, a lot. and, i wouldn't grease it up, the reason mine was stuck so badly was because my system needs a flush really badly. the orange-ish brown crud floating around in there made the monovalve stick. grease won't help that and probably shouldn't be added into the system, no matter how little.

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1985 300D - 1984 Euro 280E AMG Clone (SOLD) - 1978 280CE (SOLD) - 1983 300D (SOLD) - 1981 300D (SOLD)