overflow problem baxi 105e?

hi i have a baxi 105 e which seems to have a problem with keeping the pressure when it cools down but when it fires up it goes past 3bar it is also using the overflow for blowing off i got told to check the exspanstion vessel by taking the red cap off teh tyre like valve at the back and if it leaked water when pressed it was broke but no water just air any help please guys