I run a Worcester Heatslave sealed system. I returned recently to find system pressure 0. I top it up to the required 1 bar and fire it up. The pressure climbs up until it reaches 2.5 bar which I think is a little high. So I bleed it off until 1 bar is achieved. When the system is switched off overnight pressure drops to 0.....Thinx - since there are no leaks - is it possible that the expansion tank pipe could be blocked or are greater dark forces at work?