Tell me why ?

[mygmctruck1]

I went threw Heck trying to get my fuel gauge to read , tryed different gauge , new wire and I already had a new sending unit. When I oem out the sending unit it read per spec . I would hook up all the wires , had a good ground . I would lift the float arm and gauge wouldn’t work period.

Anyhow , took the truck in to fill gas tank to full and guess what the
gauge works !

How come I had to have a full tank of gas for it to start reading , when nothing else made it read before ? Never heard of such a thing !

Anybody else ?