During a loop, your primary experienced forces are lift (forcing fuel and klunk to the bottom of the tank) and gravity (forcing your fuel and klunk towards the dirt), in the dive portion of your loop, the gravity vector and the lift vector combine to put your fuel in the front and bottom of your tank, leaving a big airspace in the back/top. Lift force will depend on how tight your loop is and airspeed. If the klunk line is too stiff and you are not generating enough forces, you can and will suck air here, BUT NOT WITH A TOTALLY FULL TANK. That may be an easier test, fill the tank totally full, and do a loop as soon as you can.

Anyway, its an easy thing to check, I was just offering a possible solution.