Your expansion vessel may be goosed.
Depends what he meant by 'full of water'.
Was that the air volume was lost, but the bladder was still dry internally, in which case yes it can be pumped back up;
Or was there water coming out the Schrader valve? In which case it's goosed: water on the air side means the internal bladder is ruptured, and it must be replaced.
If the pressure is steadily rising over time even without head, then water's getting in somewhere.
As Beardy says, it could be passing at the fill loop: these are usually disconnectable, so you should be able to see any flow through the closed valves.
The only other source of water into the primary loop I can think of would be a pinhole in the secondary heat exchanger, where the Hot Water circuit gets heated from the primary circuit.
This would allow mains pressure cold water to leak into the primary circuit.
But I think that's what he has replaced.
<ETA: Like wot Diver300 said below.>
What's it for?
The heating primary loop ( the radiator circuit ) is a closed system, unlike older systems which were open vented with a header tank.
It is pressurised to 1 bar when cold.
As you heat the water, it expands.
Water is very incompressible, so this results in the pressure rising dramatically.
There is a safety pressure relief valve which will open at 3 bar, but the system should not be getting to that.
The expansion vessel is a small tank attached to the loop, containing a rubber bladder which is filled with air to around 1 bar. This will occupy most of the volume of the tank with the system cold.
As the system heats up, the water expands into the expansion vessel, compressing the air in the bladder.
Air is much more compressible that just water, so the compressibility of the whole system increases.
This means that although the pressure will rise at it heats up, it can expand into the expansion vessel; the expansion vessel limits this pressure rise to something like 1.5 to 2 bar.
Sometimes, the air volume just gets lost due to leaky valves, and all it needs is pumping up.
This should be done on a de-pressurised and cold system.
They are normally pumped up to around 1 to 1.5 bar, it will usually say on them.
However, if there's water coming out the inflation valve, the bladder has failed, the air volume is lost and the whole thing fills up with water.
At this point, it can no longer serve any useful function: there's no expansion air volume.
The whole system now has the very low compressibility of water, and your system will shoot up to 3 bar and then the relief valve will open venting pressure to outside.
When it cools, it will drop below 1 bar due to the vented water, and will need topping up again.