I have a a potterton prima boiler on a sealed system with a flamco expansion vessel. I have to fill the system every time I want to have any heating, and after a couple of hours the boiler won't light cos the pressure is too low! The pressure valve drips very slightly at normal pressure, but it drips constantly once the pressure goes up to about 2.5 bar when the boiler is on. Having read other posts it sounds like the EV might be the problem, but on the bottom of the flamco vessel there is a valve which seems to screw right out, not a schraeder type valve. Does anyone know anything about this type of vessel and how I can check if the diaphragm is knackered?