In the past I've had fuel tank leaks. I prevent them now by dunking them in a big bowl of water (in the sink ) and putting fuel dots or some how plugging all the lines except one, and then blowing into a fuel line connected to the fuel tank with my mouth. If you see any bubbles anywhere, it has to be fixed BEFORE you install it in the model. Really tighten down on the screw in tank stopper. The worst you can do is strip the head of the screw, but it wont leak unless the plastic is defective (very rare). Do this on all the models that you assemble.