I have dual citizenship, American and Brazilian. Can I travel to Brazil with an expired Brazilian passport, and return to the US with my American passport?
To enter Brazil you must present a valid travel document (US passport) and a proof of Brazilian nationality (Brazilian passport even if expired, or any kind of document that proves Brazilian nationality)
As recommended in the comments, use your US passport as your main travel document for the airline and present your Brazilian passport for Polícia Federal as proof of nationality. No need to mention the expired Brazilian passport anywhere else.
Depending on how close your trip to Brazil is, which other documents you have and for the sake of completeness, I see 2 additional options (besides the one on this answer) for you:
- There's a document called ARB, Autorização de Retorno ao Brasil ("Authorization to Return to Brazil"). In short, ARB it is a document valid for a single trip back to Brazil. Once in the country, it will be seized by the authorities and you'll need to renew your Brazilian passport before leaving the country (or use your North-American one). I'm sure you can contact your nearest brazilian consulate or embassy and they will provide you one.
- If you do have your Brazilian ID with you (Carteira de Identidade) you could use your US passport to enter a neighbouring country (Uruguay, Paraguay or Argentina) and from there use your Brazilian ID to enter Brazil, since it's a valid travel document for trips within those countries