Joe,
If the float drops because the fuel in the bowl is low due to evaporation, the vacuum is broken and the fuel in the line can drain back to the tank if the check valve in the fuel pump is bad. It will not, as you say, siphon the fuel out of the bowl.
And I have checked my tank and the top of the tank is lower that the fuel pump. If your tank is behind the seat it may not drain back unless the tank level is low.
Kurt
