I have an old system but a boiler and pump approx. four years old. During its life, the system has worked perfectly until a few weeks back when the water began bubbling loudly through the pipes and I noticed one or more radiators were unevenly warm. I have vented regularly but found now that often no air or water will come from the radiator. There was a large quantity of sludge (which has now been cleaned out) in the feed and expansion tank but still we seem to have very large volumes of air in the system. When the heating is switched off the hot water bubbles through the pipe; when the hot water is used, ie. shower, then more bubbling occurs. Please can someone give me some accurate advice? Why can I not get water from the bleed and often not even air when it is obvious by the loud bubbling, gurgling sounds that a large volume of air is present? Help! No leaks are apparent anywhere - yet!