Do not pump full tank of petrol

Many of us are not aware that the petrol kiosk pump has a return pipe-line (in black). When the petrol tank (in the car) reaches full level, there is a mechanism to trigger off the pump latch and at the same time a return-valve is opened (at the top of the station) to allow excess petrol to flow back into the sump. But the return petrol has already pass through the meter, meaning you are donating the petrol back to them...