This is more likely to be a perforated diaphram on the expansion vessel.If the pressure goes up rapidly when the heating is on that is the most likly cause.
Sig Lofnes, November 2006
Had a friend with this kind of problem, and it turned out to be the expansion vessel had no pressure in it. Get yourself a car tyre pump and put in some air, maybe 6 pumps, and see how you go. The expansion vessel should have a car tyre type valve in it.
Dode, December 2005
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I can answer this question...
boiler looses pressure when heating not on ,but no leaks?
