How fuel pumps stop overflowing

When you pull the trigger fuel flows through the pipe into the nozzle

While this is happening air is passing through a narrow channel to flow in

Once the cars tank is full the rising fuel level covers a tiny hole near the tip meaning no air can get through

This causes a sudden vacuum to form within the system that lifts a diaphragm releasing a steel ball and instantly relaxes the trigger mechanism

The main valve shuts with a click and the nozzle automatically stops pumping