loss of pressure in brittany combi 80?

I keep losing pressure in my combi boiler, i recharged the system and when i put the heating on it seems to slowly lose pressure over about 8-12 hours, but if i run the hot water it will lose all pressure within a matter of minutes. I have charged it ti 1.5 bar then closed of the heating inlet and outlet valves and cold water supply in but the pressure still drops. The only thing i can think it could be is either the heat exchanger has got a leak or maybe the diaphram in the Expansion Vessel has a leak. Although i have checked the expansion vessel by ckecking if air or water is coming out on the valve and it sees to be air, but then if it is slowly filling up i will not know till it is full of water i suppose. just wondered if anyone could help.

Cheers