How can I fix my Halstead Ace High boiler?

After having been so impressed with this website over replacing diaphrams I was hoping someone could also help on another issue. I have to top up my Ace High boiler every 2 or 3 wks so obviously have a minor leak somewhere (although where is anyones guess) have put bowl under overflow to check that. But even when I have topped it up to 1 bar within a few days the pressure is zero when off and rises to around 2.5 to 3bar when on. My question is would such large variations in pressure have anything to do with the expansion vessel? Hope some one has similar experiences, thanks in advance.