Hi, got a bit of a problem here, drained down the oil combi boiler to fit a new expansion vessel and refilled to 1 bar, set it to just hot water and let it run to heat th storage tank after five minutes the pressure started rising quite quickly and it switched off.The pressure continued to rise until the pressure release valve kicked in hissing slowly, with the pressure still rising I manualy operated the valve and a big whoosh of steam came out, I held the valve for a long time until the pressure went back to zero and still no water out the valve.So what's happening, air lock?Thanks for any help guys.