Originally Posted by
boboqui
My guess is that departing Brazil using your US passport won't be a problem (unless the computer flags that you are a Brazilian citizen...). On the return, you'll only be allowed to board if you have a valid visa, but once you land, as a Brazilian citizen, you can enter Brazil without a passport.
That is innacurate, as they would look for evidence of getting into the country with the US passport.
They are surprisingly picky on this: I asked that for my son, since a Brazilian passport for a babies below 12 months is only valid for 1 year (as opposed to his Swiss passport, valid for 5) and was told that he could only leave the country with his RG only , if travelling to Mercosur. That means he's fine to see his grandparents in NZ, transitting through EZE or SCL,mbut not his other grandparents in Switz.
Anyway, long story short, you should be OK if you bring all the documentation, as this is clearly an extenuating circumstance, but I would give myself plenty of extra time (i.e. arrive at airport 30-60 mins earlier than you would normally) in case you get an exceptionally pedantic/ignorant immigration agent.