Potterton combi 80 boiler?

I have re pressurised the expansion tank as suggested. There is no water coming from the valve on the unit nor any wetness around the rim. The problem stays the same with the bar guage rising to the red section when the heating is hot and slowly falling to zero red area when cold so I have to top up the water or the pump makes a growling sound on turning on the system. Have been told it may be a pressure leak in the pipe work so have put leak seal into the system still no changes so stumped, please help if possible.