The air release chamber has a float and needle valve within. Cause of problem in my experience was lime scale build up in the chamber preventing the float from rising/falling with water level and thus close the needle valve when required. Suggest drain system, unscrew top of chamber ( like the top of a jar ). Float and valve lifts out with top for cleaning. When refitting make sure the small plastic cap on the top of the cover (like a tyre valve cover) is NOT TIGHTLY FITTED leave 1/4 of a turn from fully tight. Refill system.
Bill, February 2005
