If It is tank fed then you may have an air lock. Try back filling by placing a hose pipe from the drain off point to your kitchen sink cold tap. The pressure should push any trapped air into the radiators (usually with a whoosh!) and you can bleed it there. Apart from that procedure make sure all your relevant valves are open to fill and that your drain off point is closed too.
Pete the Plumber.
December 2010
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