Rick, the cessna fuel system is a simple one yet an 'obvious' problem can still be hard to nail. Lack of replies suggest that your engineers would have probably found and corrected the problem by the time someone attempted to answer your question.
The 'air pressure' system I presume is the tank vent. It should have a check valve in it ie, it will let air in to replace fuel used but should not let air out. If it failed in the closed position you may end up with the problem you described.
Also the vent can be blocked by foriegn objects, one example we get here in Oz is mud wasp nests if the vents are not covered when the A/C is parked.
If your tank bladder had already semi collapsed, lets say from a previous flight , the next fill will give a higher reading (on your qty gages) than what is in your tank. Therefore a lot less endurance.
Good Luck.