Originally Posted by
Mats1985
If ya take of the cap when it's running the water flows proply just when it turns off it bubbles and pressure
is it a metal header tank or plastic?
I had a bailey one, kept doing this, changed the cap and all was well, either way might be worth checking it to be sure.
Check what i said above re stat..and perhaps give it a bleed again to be sure, if still doing it then its looking like a possible HG prob.