With my vast experience of goldfish bowl cleaning and having watched 'Finding Nemo' a couple of times, this reverse siphoning effect going on in the 135 fuel tank seems a bit far fetched. But I guess that's the stage at which we find ourselves.
The source has to be higher than destination. We have been told that the transfer pipes run to the bottom of the supply tanks, the same level as the transfer pump inlet, add a couple of obstructions to a smooth flow, such as faulty nrv, non operating pump, a couple of sharp turns or a pocket of air and syphoning simply can't occur either way.
Having said that, anyone know if the diameter of the tubes is constant or are there different sizes of tube in the transfer system?