Combi boiler pressure loss

[Jborgy]

Have an ideal logic 35 boiler, logic.
Was loosing pressure several times per day on water and heating but just had new vessel and PRV fitted, now water fine, but on heating would be ok while heating on, rads hot and pressure ok, when heating stops and fan runs for 5mins then boiler stops the pressure starts to drop, and 2 mins later no pressure have to fill loop.
Rads been vented, no leaks can see or feel.

 

[jj4091]

Diverter valve leaking? Was it serviced before the expansion vessel was changed. Is there any signs of water dripping from the bottom of the boiler. Has the new expansion vessel been pressurised?