My central heating pressure keeps fluctuating. When switched off, the pressure sits at 1bar but when the CH is on, the pressure rises to 3 bar and is tripping the heating. It is a closed system with a flexcon Cubex expansion vessel. I was guided to try and re-pressurise this and have had no joy. I bled the radiators and switched off the water supply, to bring the water pressure to 0. Testing the air valve nothing has come out (I assume this is causing the issue) I tried re-charging with a foot pump but to no avail, the system would not take any air in. I have stripped the shrader (spl) valve and replaced but still have had no joy re-charging. Any advice would be greatly appreciated.