the round bump on the top of the handle actually contains a little diaphragm connected to a second little tube that, once you’ve got enough in the tank, begins to suck up overflowing gas, and tells the diaphragm you’re full.
see: https://www.youtube.com/watch?v=TFKOD3KRkZs
so it comes down to the handle knowing how much pressure is pushing back against the pump.
Latest Answers