Excellent news
Glad it’s going better
Put a vacuum gauge on it today. Conclusion:-
The vacuum required to open the Rochester valve is 2-3 inHg. Reasonable to assume therefore that the tank vapour pressure required to open it is also 2-3 inHg and that the tank must have been designed to take this amount of positive pressure. The vacuum delivered at the valve via the hose from the inlet manifold is about 15 inHg at engine idle - so manifold vacuum will clearly be more than enough to open it.
The tank pressure must therefore be 3 inHg or less - escape of vapour on removing the filler cap presumably seems more because of volume rather than pressure. So - no problem.
Frankie