actually "chui brésilien" is correct, you cannot write it at school, at work or whereever, but you can say it because it's a lazy way to say "je suis brésilien !" so with your friends don't hesitate : Salut les gars, chui brésilien.

Later

Je suis brésilien/ne.
Chui est correct mais fort familier.

You have to write "Je suis brésilien", but most people will pronounce it "Chui brésilien", except in formal situations.

This is because of the "dropped e": je suis = j'suis (pronounced "chui".

Je suis brésilien/ne.

merci