I am having a similar problem. I Bled the upstairs rads while heat was on. Two of the rads bled air and then the hissing stopped, but there wasn't the expected trickle of water. They seem to heat OK though. However there were then gurgling noises from the pipes and the rad downstairs closest to the boiler stopped heating. There is no heat on pipe into it or out of it. Is it possible for air to be in the system? if so can I bleed it myself, or will I need an expert?
Secondly, I cannot find the expansion tank. Would this be inside the larger tank in the attic, or could it be elsewhere? How full should it be?
Finally, would it be recommended to remove and drain the rads yourself (I have another probelmatic one), to remove sludge or should I get an expert. Seems to me to be a case of closing the values, readying a bowl, remove the nuts and lift, then replace the nuts and thred them over a bit of plumbers tape?
Any advice appreciated as always!