Hi You do not say if you can see where it is leaking from? or do you not know, if you do not then it may well be the pressure vessel that will need changed, as when you refill the system and the boiler comes on the pressure has no where to go ( if vessel faulty) so blows the extra pressure out through the overpressure relief valve and will work ok till the unit goes off and cools down losing pressure, so I would suggest you change the vessel.
hope this helps. :)
hi james the boiler leaking will be drawing in air thats why your having to bleed the rads every day you must find the leak and repair it as soon as possible as air getting into the system can cause numerous other faults to develop start by looking in all the visible places first ie the boiler check union nuts on heat exchanger /pump ect: then check all rad valves for leaks if nothing the leak will be on pipework which can be harder to find if only a small leak but if boiler is dropping to 0 qiute quickly then the leak shud be pretty easy to find tip:check ceilings for staines or any new cracks gud luck bazza
try to locate leak by checking pipework rad valves and boiler, if no luck and the pressure isn't dropping too quickly buy leak sealer to add to system (will only work on small leaks). This can be added using an injector bottle or by removing and adding it to a radiator.