I have a Vailliant eco tec pro 28 boiler and the pressure is dropping overnight whether the boiler is on or off from 1.5 bar to 0.9 bar and turning off the heating. I cannot find a leak anywhere in the house unless I have missed it. I do not think it is the expansion vessel as the bar is not rising to 3bar. I have read it could be the PRV valve so tonight I have put a bag over it to see if water goes in it. I have also filled the boiler up to 1.5 bar and shut off the flow and return valves to see if it is on the system side. What my question is if the flow and return valves are shut and it is not on the system side will the prv still give out if it is that or will I have to try again tomorrow when the flow and returns are open ?