What's going on? (brake question)

Thanks guys..
So I dedicated the entire day today to my car..But since this morning, I haven't been able to bleed the brakes. They hold pressure when everything is sealed up. And at 3 of the four wheels, fluid comes out when the bleeder gets cracked open. On my rear right wheel, where I need to start the bleeding, I have fluid at the brake line, but not at the bleeder valve. And even when I hook up my mityvac to it, nothing is coming through.. its almost as if something inside that drum is plugged up between where the brake line comes in, and the bleed valve...If that makes any sense. somebody please chime in..

I think if you take that bleeder screw all the way out and you'll find it plugged with rusty crud.