Was there an upper part(s) of the pressure plug that was removed before the photo was taken? It looks like what is shown is the lower cone and the "brake shoes" (as in what a drum brake would have, which is what pressure plug mimics). If so then the lower cone needs a tap from a punch and hammer to break it loose from the "shoes". With the poor photo it's hard to say if the brown stuff is corrosion, dirt or dried on food/drink gunk Andy