Dan, your explanation is theoretically correct, but Newps wrote from
experience. With an incorrectly located vent, and both tanks full, fuel is
forced from left to right. The right tank stays full, and usually dumps
fuel out through the cap, which has a vent that opens both positive and
negative (Monarch caps, at any rate).

I have been maintaining Cessnas for ten years now, and the
proper Cessna cap will not allow air out of the tank. By law, an owner
MUST use parts as per Cessna parts manuals, and any other cap that
isn't PMA'd for that airplane is illegal. There is no way that the
airplane is legal if it's doing that, since it doen't comply with the
manuals.

Dan