Gas moving from back to front tank???
 

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Well, yes....
In my excitement I neglected that information...
78 F150 (as shown above)
Twin tanks (is that standard?)
460 Auto (400 original according to vinwiz)
4x4

Does this mean this CAN happen? Are the tanks connected? Why oh why?

crickets....

Well...
nobody knows... so I will go somewhere else. never to return.
if
anybody
cares.
(dont bother answering... I wont be checking)

Well if you do so happen to still be listening, Im not sure for your specific model but in my friend's 1996 f150 the fuel return feeds to the front tank, so if you are running idle in the rear tank you will essentially be pumping fuel from rear to front.