This sounds like the red exspansion vessle requires re-charging.
Open the valve near the vessle (this will say 3Bar on it) to middle position any water left in the system will come drain outside, you will require a foot pump on the vessle you see a car wheel type valve remove the dust cap and pump the vessle to 1.0 Bar which slightly higher than needed but you will loose some pressure when removing it. close the PRV valve and fill the system as normal.
David
October 2006