Just an update to this thread. I fitted the push button bleed from Best Fittings to my 7L tank, it works a treat. Just make sure digits are kept clear of the exhaust port on the rear.

Although not essential, I tend to operate/test momentarily (the normally closed valve) before opening the main valve on the HPA tank (to ensure the bleed is always free to move/open).

I noticed that the button itself screws off (for assembly/disassembly) which suggests it would be possible to replace any internal seal (if appropriate) in the long term.

The 'pop' is a lot less when directly filling a cylinder, whereas the pushbutton bleed on my Day Tripper bottle has quite a pop when bleeding a microbore hose and adapters.