It's not "refilling", the bubble just went to the highest point. It's all uphill from the tank, and the float valve is a suction breaker. If /when the float opens, the air exits, and the fuel runs down to the low spot. And the same thing happens on the supply side; the air travels up to the highest part between the pump and the tank, some to the front perhaps and some back into the tank, or out a crack in the line or a where-ever, and whatever fuel was in the line runs downhill towards the lowest part usually under the passenger seat. Then when you restart it, the pump primes itself and pushes the gas up to the carb, and the air goes out the floatvalve, until the gas gets there. And the semi-transparent filter is the witness. Now this creates a low pressure in the supply line, and if the fuel level in the tank is high enough, it will travel up the pick-up over the top and on towards the front. But if air gets in there, then , well, you know what happens ,then.
