Try to re-commission the vessel first:

Discharge system pressure.
Push in the valve and remove air pressure in the vessel.
If water is discharged from the valve, remove the schraeder core and connect a gauge tube.
Water inside the vessel may be condensation and not the vessel diaphragm split or leaking.
- Note: Condensation may be as much as 0.5 litre.
To check - increase system pressure, slowly, to a max of 1.5 bar to remove any condensation in the vessel.
If the diaphragm is split water may continue to discharge - change the vessel (see Useful Tips below).
If the diaphragm is intact then release the pressure from the system through the drain-off.
Renew schraeder valve core and connect foot pump.
Re-charge vessel to 0.7 bar (cold) or manufacturers stated pressure ensuring the drain-off is open.
Leave the foot pump attached for 5 minutes, if the diaphragm inside is split or damaged the pressure will drop, if it stays stable then the vessel is serviceable.
Close drain-off and re-pressurise system to 1.2 bar (cold) or manufacturers stated pressure.
Check schraeder valve with L.D.F.

Useful Tips

If the core is seen to be leaking and it's not possible to replace it due to its position it is acceptable to fit a Flexible Valve Extension which can be left attached to the vessel as the new schraeder valve and makes future repressurising more accessible.

When pumping up the expansion vessel if the foot pump gauge rises very quickly (after 2/3 pumps) then it is likely there is a blockage or restriction. This is more than likely to be in the pipe or hose to the vessel and replacing the vessel won’t solve this problem. The pipe/hose will need to be removed and unblocked.

If the vessel does need replacing then consider fitting external one eliminating the need to take the boiler off the wall.