When ever I have seen an expansion vessel installed it been with the valve uppermost, with the inlet at the bottom.
Surely this will trap air under the rubber diaphragm ?
When the system's cold the diaphram presses down to the inlet by the air pressure behind it (the cold pre-charge) and self purges any air that may have been caught when the boiler was running and the system was hot.
In a combi or remote EV system, the vessel wouldn't normally be the highest point in the system, there would usually be an auto air vent above the EV to vent air from the boiler circuit that would also minimise any air that would enter the EV.
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