In a nutshell
If a pickup is sucking air then clearly it is not fully immersed in fuel.
Looking at the schematic, if a 135 were steady on the ground in cruise attitude and the forward transfer pump is not functioning, when the fuel quantity in the main tank drops to ~95 l, no fuel will be pumped to the supply tanks. The supply tanks will run dry and the engines will stop.
What happens when the aircraft is flying and fuel is sloshing around? Partial fuel prior to stopping?
What might a pilot do when he thinks there is sufficient fuel aboard but the engine(s) falter minutes from base?