I read some advice on this forum (thanks everyone) and checked my expansion vessel faults to find something that I think may be worthy of comment / question please? I have a bungalow with a 15 year old WB 28i junior with the pressure fluctuating (0-3.5 bar). I put a pressure gauge on the Schrader, water squirted out and it read the same as the system pressure (failed). I drained the vessel but realised it was difficult for me to change it (behind rear panel). So, I re-filled the system to 1 bar and re-checked the vessel pressure (also 1 bar). Tapping the vessel, I think it's now half full of water / half air and it appears to be working perfectly well (1-1.25 bar) albeit with a failed vessel. It appears that the vessel became full of air when I emptied it and became compressed to half full of air when the system pressurised to 1 bar above ATM (P x V = 2P x 1/2V). Why does the EV need the balloon (other than long term corrosion)? Why isn't this a similar principle to using a temporary empty radiator? Sorry if this question has already been answered!