This used to be used as a short term fix to get people through the weekend on a Friday evening. Not a long term fixAs far as a new expansion vessel goes, there is an alternative. You can drain the system and after refilling, partially bleed one radiator, preferably in a room you don't use much. This will act as an expansion vessel. As the pressure relief valve is a purely mechanical device I suggest yo dismantle it, give it a good clean and retest.