Finally got my Daystate Huntsman Classic from the RFD today, it's been in storage since lockdown began. I'm having a problem filling it though. I can fill it OK from a hand pump, no issues there. When I use an air cylinder, it fills the tank, but the fill valve appears to stay open, so when I bleed the cylinder before detatching it, all the air comes out of the gun, ie it bleeds the gun's whole cylinder.

A previous Huntsman which was in my possession for only a short time did the same thing, so I assume I'm doing something wrong?

Any advice appreciated.