How many hours would you be in GRU? There is a max (7-9 hours, I believe) for transiting, without a visa. I think most flights from/to the U.S. have a turnaround time of more than that (more like 14 hours, or more, I believe). The fact that you are returning to the destination of origin may make it even more complicated for those looking at your tickets and deciding if you need a visa, but I'm not sure. Better contact the Brazilian embassy and the airline. You want to make sure the airline will let you board.