Corrected entry: Peter was the only Brady kid who never cried onscreen, though he did come close to crying in the episode "Confessions, Confessions."
Correction: In Season 2 Episode 16 ("The Drummer Boy"), although we don't physically hear him cry, we see him walking away appearing to cry at the end of the scene after his football teammates ridicule him about singing in the glee club.
Correction: He did cry right after that. Carol hugged him while he was softly crying.