Hi
if the leak still persist after a blast of about 150bar,then it’s probably the O ring in the gauge that is hard and not sitting correctly on the valve,hence letting air out,if you can fill for a short period,dunk the gauge end of the cylinder in a glass of water,this will confirm what I am on about.
its an easy fix,please make sure all air is out of the cylinder.
you have to buy a 36mm hex bit to undo the gauge head,once undone undo the silver slotted bolt to release the valve,you will see a O ring around a brass collar,take the O ring of and lube before replacing with