Never heard of anyone ever having that problem, it is sort of like worrying if the engine oil would spit oil back into the breather. If this was a real concern I think they would have problems selling the kit. No problems with the kit that I have ever heard of. It is a long ways up to the breather hose from the bottom of the frame. If you are really worried about it then anytime you change fluids, disconnect the hose and drive it down the road to make sure there is no over fill.