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Make sure the cap on the Schraeder valve on the old expansion vessel is tight.
Though a failed or de-pressurised expansion vessel is the most likely culprit when water passes out through the safety pipe it is not the only possible cause.
If the plate heat exchanger (secondary heat exchanger) fitted to most combi boilers is leaking internally water will probably be forced out through the safety valve and the pressure gauge will read consistently around 3 bar.
You then heat the system up and the pressure rise should not be excessive. This pressure should drop back down as the system cools.
If the expansion vessel is damaged you need to change it if you can get at it.
If your expansion vessel diaphragm is ruptured there is a temporary "fix" you can try.
Drain water from a drain cock and release a radiator bleed point (upstairs) to allow air in to the radiator.
If, however, the vessel pumps up correctly, you re-fit and tighten the Schraeder valve cap and then fill the system water side again to increase the pressure.
Typically you want between 1 and 1 bar in the heating system when the system is cold.
Allow quite a lot of air in so the radiator is only full of water (you'll have to guess)…Close the radiator bleed and the drain cock and re-pressurize the system.
Make sure that at least one of the radiator valves is open.