If you think about it the upper seal is not really a pressure seal, 22 psi on the upper seal is more than enough. If you don't seal the bottle o/flow what pressure does it hold? It should obviously be 16 psi. I postulate that you have more going on than a cap with an upper seal that is not sealing;
a) If the cap is leaking then the lower seal/spring is not sealing or you have exceeded 16# pressure in your system. What I suspect is the neck may be too long on your bottle for the cap you are using. This is a common problem with oem bottles as they are euro necks and caps here are US length.
b) when the pressure seal opens it allows coolant to flow to the o/flow bottle, which is a non pressurized bottle. You don't have a pressure cap on the o/flow bottle do you?
Let us know what you find.
Julian
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