Boiler keeps over presureising?

I have a main 24he and it keeps building presure and releasing throgh the presure relif valve. I have replased this valve but the fault came back. I have checked the filling loop and this is not letting bye. So I then replaced the expansion vesal and refilled to 1.5 bar of air and then refilled the bolier to 1.5 bar and left overnight. On checking today the presure is back up to 3 bar and letting off through the presure relief valve. I have released this presure and reset to 1.5 bar, any ideas?