AoF, thanks for filling in my blanks as I was to quick. There are no caution for us pumps. Only when swithced off.
What I´m saying is the overflow (both ways) correctly fills up the supply up to a point when transfer pumps are off or check valve us. As you know the pumps have a dry operating time of Approx. 20min. Multiple dryruns could have damaged the pumps over time. Anyway, this means it will still be fuel left in the main tank even if supply tanks are running empty.
AC attitude does of course make a difference, but if for whatever reason both transfer tanks are off, what is then transfering fuel to the supply tanks?
Prime pumps are in the supply tanks and makes no difference if supply tanks are empty.
I know this is a long shot, and it is unlikely, but system failure do occur and is a possible explanation to something happening so quick.