I have a separate tube and inline valve I attach for counter pressure filling - and have clearly forgotten on this picture to replace the blanking plug as you are quite right
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Short version, yes, long version it depends on the check valve type how much that pressure drop is. All check valves rely on a pressure difference between the inlet and outlet to keep them open, if there wasn't a pressure difference then the valve would close, so by definition there has to be a small pressure drop across a non return valve. There's a difference between the cracking pressure which is the amount required to open the valve in the first place, and the pressure at which the valve closes - which may be reverse pressure. The non return JG inline valves I've seen are diaphragm check valves with springs, so there is a constant pressure differential required to keep them open. How much is likely to be very little, I've seen 1.5psi quoted but that was for a different form of valve.