For what it's worth, the old BOC test for pipeline Entonox demand valves did not worry too much about leaks ("... listen carefully at the patient outlet for leaks").

Instead, it recommended two "breathing resistance" tests - which, to my mind, are far more important from the patients' point of view.*

Both relied upon the afore-mentioned test bellows (BOC A-frame).

Quote
Connect a hose and handpiece and fit a test bellows in place of the facemask ...

1) Slowly expand the bellows and check resistance is within the range 0 to -2.5 cmH2O ...

2) Expand the bellows quickly from the closed position and check resistance is within the range -20 to -35 cmH2O ...

* I always liked to check-out kit (of all types) in a fashion that mimicked as closely as practicable actual "patient usage" (not too much fancy test kit about in those days).

Notice how low those negative "breaths" are:- 1.8, 14.7 and 22 mmHg respectively.

If you don't inspect ... don't expect.