mmm- hi-spec is quite correct but re pressurising expansion vessels requires an accurate gauge and if it is faulty then it will lose pressure and it will not keep the half bar or whatever pressure you have pumped it to..
If your pressure does go to 3 bar when hot then hi-spec is correct in saying that the vessel may need recharging or replacing.
but -- you say you have a leak - is this at 3 bar or lower?? if you have a leak on the boiler check the pressure relief pipe-work (it goes outside- or should do)-- it may leak intermittently.
But if the boiler is leaking then you have the answer - find leak - cure it.
One more thing - it is known that many systems can entrain air (results in loss of system pressure)-- you could if you know how add Fenox LS1 to the system
get advice from merchants- This will help stop any small leaks on the system - but I would check the boiler carefully first.
dave broadbent
June 2007