The system sounds like it might be suffering from corrosion (sludge). I assume it is fairly old?
Probably best to introduce a cleansing chemical such as Fernox Superfloc or Sentinel X400, operate system for a few days, then drain down and re-fill. Add 1 litre Fernox MB1 or Sentinel X100 Corrosion-Inhibitor when re-filling system.
CMac
November 2010