From the front of the tank we have the fuel pickup line, it should go to the nipple in the body of the carb. It should not have a T in it.
Working our way back, we have the return line. It goes to the nipple in the carb near the fuel bulb. It has a vent line for the tank teed into it. This line only returns fuel when you pump the bulb. That is why you can use it for a tank vent.
The last line, at the back of the tank, is the tank fill line. I have mine plugged when I am not filling the tank. It can be used as a vent, but because it doesn't have a loop higher than the tank, it can drip fuel out during aggressive flying. It can stain your rear mounted muffler, if you have one.
I think the tanks are just thin and they don't hold their shape well.
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