when refueling a propane tank, how do you keep from forcing some air into the tank? I know there must be a way to aviod pushing that is in the fuel inlet into the tank. there would only be a little air in the inlet, but after several time filling the tank, the little bit of air put in each time would need to be bled off. How is this done? I know that only 80% of the volumn of the tank is filled with liquid, the other 20% is vaporized propane. You can not just open a valve, or the liquid would vaproize with the loss of pressure. Does anyone know?