OK, so a couple of points...
1) When pumping up expansion vessels, you can't just 'have at it' straight away.
You need to have the water side open, so that the bladder can push the water out and expand to the full volume of the vessel at the desired pressure.
It's not just about pressure, it's also about volume.
If the expansion volume is lost, then the vessel is mostly full of water.
If the system remains sealed, then you are pumping up a tiny air volume which is no good.
So you need to shut off the cold supply, and open a hot tap.
Wait for the tap to stop running ( ie the expansion vessel has expended itself ).
Then, as you pump up the air-side, the water in the vessel is expelled via the open hot tap.
This will take more strokes of the pump than if you left the system sealed when inflating it.
Once the bladder is up to pressure with the water-side open, close the hot tap and then open the cold feed.
There will be a short hiss of water as the water side re-pressurises.
2) Cheap PRVs will often fail to seal after they have lifted a few times.
I suspect if you re-inflate the expansion volume as I described in (1), you might be good to go.
