Pressure loss

[chris2003]

Hi
Desperately in need of some advice regarding my Baxi Combi 80E boiler, over the past year or so my boiler has needed the pressure topping up but now is becoming more frequent so yesterday i topped the pressure up to 1bar and then closed the flow and return valves to isolate the boiler from the heating system, I left the boiler off for 24hours and checked the pressure and it has virtually dropped of so I assume the loss is somewhere in the boiler. I assume what I have done has eliminated the possibility of a leak in the heating pipework etc as there is no evidence of leaks anywhere in the house.

Any advice will be greatly appreciated.