In the old days occasionally a steel pick up tube would rust through so once fuel level dropped it would suck air even with half a tank full. More so with steel tanks where the pick up pipe was separate.That comment about half a tank rings bells in my distant past but I cant summon the thought yet... Split fuel pickup pipe ij the tank?
Rusty tanks were quite common, I recall a Mk3 Ford Zephyr I had that I tried to blow the fuel line back to the tank and forgot to remove the filler cap, the whole of the bottom of the tank went porous and petrol flooded out, fortunately spares from scrapyard were still available for that model in those days.