Vokera Mynute 20SE boiler?

I got some useful feedback to my previous question about large varying pressure in my boiler and the fact that if I charge it up to 1.50bar then overnight it drops to 0.50bar (there being no obvious leaks). Terri suggested re-pressurising the Expansion Vessel in the boiler. When I looked at the bicycle type nipple he described I noticed slight bubbles of water/air coming out and when I pressed the nipple there was a definite flow of water (not air). Does this indicate that the Expansion Vessel/diaphragm is knackered and should be replaced? or is it worthwhile still trying to re-pressurise the Expansion Vessel to 1.0bar?