I just replaced my MONO valve insert yesterday and yes it seemed hard to pull up. I got the feeling it was magnetized? Someone more familiar with this valve could hopefully verify this. I know the coil produces a magnetized field to raise and lower the diaphram but I would think this would not be the case with the key off. I've seen alot of these units very rusty upon disassembly in wrecking yards. That would cause stubborn removal as well. Its best to remove this insert with a cold engine and the radiator cap off, no pressure. Less coolant loss.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.