I have a potterton 100 and when pressurised to 1.2 bar as normal when heating is switched on pressure rapidly goes to 3 bar PRV dumps outside. I drained upper system found shraeder type fitting and pressed needle ,air came out no water! there is a flexible pipe running from pump pipe connections to expansion vessel i removed this and cheked it wasn't blocked,replaced it charged up expansion vessel to 1 bar and noticed it dropped to 0.7 bar, pumped back to 1 bar same happened it dropped to 0.7 bar.filled system bled correctly and boiler still the same, overpressure prv dumps. i assume the expansion vessel is no good! I can mount another vessel same capacity right next to boiler but can i just remove flexible pipe on old vessel and connect this to the new vessel alongside the boier and fit new PRV the flexi pipe will reach no problem.