it is the expansion vessel.to repair open pressure relief to zero ch pressure.on the vessel is an air valve like a bike or car tyre valve.attatch a foot pump or similar device and pump up th epressure to 1-1.2 bar ON THE FOOT PUMP GAUGE NOT THE CH PRESSURE GAUGE.REMOVE PUMP AND REPLACE DUST CAP ON THE VALVE.PRESSURISE THE CH PRESSURE TO 1 BAR AFTER BLEEDING SYSTEM OF AIR AND TURN ON CH.WATCH PRESSURE GAUGE it should not go .if it anywhere near 3 bar.if it doesnt its fixed if it does youll need new expansion vessel.hope this helps
cp
October 2008