pound a large flat blade screw driver into it. (not through it tho) crank it out. i used a adjustable wrench on the handle of the screw driver to turn it out. some are torx too, not a allen head. so if the allen is spinning take a better look for the star points.
once air gets to the coolant, a brown sludge builds up. at the rad cap, the over flow tank. and so on.