I have a glowworm 30ci. For a little while (several months) it has been loosing pressure, primarily when off but sometimes when running. I have had a plumber out and he pumped up the expansion vessel, which worked for half a day. At the time it was dumping water whenever on, and this continued until recently. The dumping of water has now ceased and I am still losing pressure. Also the pressure can not be resumed by simply opening the valve. The only way is to pump up the expansion vessel. This has worked until today. There appears to be air pressure in the vessel but this is no longer giving me the green light. I can not see any evidence of water or air leaks. Has anyone got any idea's please