Brittony Combi 80 pressure problems?

I have a brittony combi 80 boiler and it keeps losing pressure. When i recharge it it then seems to rise up to about 3bar then drop back down to 1.5 ish. After a couple of days drops back down to zero. have checked the Automatic pressure release valce to see if that is dumping water but it sin't. A gas man said it is a air buble and will eventually go, but surely if it is an air bubble to would not increase the pressure, just a thought??

please help