pressure loss on combi boiler?

I have a baxi combi boiler 105e which loses pressure rapidly can go from 1.5 bar to zero in the space of around forty minutes. Thought it might have been due to leaking glands on radiator but have since been rectified and no longer leaking. There are no obvious signs of leakage and only few drips from warning pipe as usual. Any ideas what could be happening or what i could do as dont want to go pulling up floorboards etc? Any advice would be most appreciated, thank you.