how to fix baxi 133 he?

hi there i have a baxi 133he combi it all works fine apart from the pressure gauge sits at zero .then when the heating is switched on the pressure goes up to one and a half bar and sits there no problem but as soon as the heating goes of the pressure drops slowly back to zero ive tried topping it up but it always seems to drop back to zero ive tried bleeding the rads but there is no air in any of them just water theres no visual leaks ive had a new prv fitted and a new expansion vessel fitted plus new pressure gauge .if this boiler had no gauge you would not think anything was wrong with it the whole system is only 2 years old pipes as well its driving me mad cheers daz