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The US citizens are required to have expensive visas to Brazil in "reciprocation" for US visas for Brazilians. Even if there is a stopover, a transit visa is required.
As far as I know, that the only exception of this rule is SΓ£o Paulo-Guarulhos Airport. If your stopover is there, and you don't leave airport (don't try to go trough customs), you don't need transit visa. But only in this one airport.
A friend of mine was travelling from Miami to Buenos Aires. He was misinformed by his airline and had stopover in BrasΓlia. He was deported for not having a transit visa.