The bulging and 'sucking in' of the O ring should not happen. I think the cap and O ring will fix your problem as long as the issue with the O ring is not due to over tightening the cap. Once the O ring is deformed it is not going to seal properly.

You do realize the 25NM is 18ft/lbs?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas