I always use fueler valves and only have two lines into the tank. If you have three lines into your tank and you're filling the tank completely then fuel will eventually come out of both the carb line and the muffler vent line. There is nothing there to stop it. Sounds like your system is fine. If you install a fueler valve then you would only have two lines into the tank. One for muffler pressure (bent up towards the top of the tank) and the carb line with a clunk that attaches to the fueler valve which, in turn, attaches to the carb. With your system, the way you describe it, fuel should come out of all lines when the tank is full. You would then plug the fill line and as soon as you turn the engine over your muffler pressure line should push any fuel in that line back into the tank. With both systems it is normal for fuel to come out of the muffler when the tank is full.