Gel wrote:
Sounds like air lock if youre sure lbleed nut fully unwound.
Leave system on to pump water through, and take bleed nut all way out
and wait.
Be ready to fit back in as soon as you feel heat at top of rad/hear
gurgling, but leave open until you see some dirty water dribbling out.

So if you miscalculate and the water starts spraying out of the
completely unscrewed bleed nut, how are you gonna get that bleed nut
back in? Even small amounts of water pressure can be really hard to
fight, I've been there...