I forgot about the check-valve, the GTS also has it. However on the GTS you just pull the carpet back and remove the screw cap on the top of the tank. On the GTS, the tank level shouldn't make a difference. Pull up the bottle slowly and it will drain. The pump has a secondary in-the-bottle outlet that is supposed to keep the fuel level up in the bottle, therefore, whether the tank is low or not, the bottle will contain some fuel.