Is it one of those 400s that has a pressure relief valve built into the front cylinder plug? You can tell by the little weep hole drilled radially into the flange part of the plug.
When these fail, they usually vent the top part of a refill then stick at say 160 bar and "heal up" until you fill again. They are meant to blow off at about 200 bar to prevent over-pressure in the cylinder but the O rings go hard after a while.
I've only got one 400 left with this device in it; all the rest I've changed the front plug and done away with them completely. They were never a standard fitment.
Other sources of leaks are the filler itself if it's the pointed type, the earlier one. More rarely the O ring under the block that carries the pressure gauge is at fault. If it's an AT rifle it's a beggar to fix too.