The problem is no air coming out. It doesn't sound like sludge. Sludge will normally give you cold spots at the bottom of the radiator. Are you doing this with the heating off? There is a danger that if the pump is running you can actually suck air in.I can take the bleed screw completely out of this radiator and no air or water comes out.
If it's a rushing noise then you still have air in it.I can hear water running through it now
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