Brandao scored the only goal of the game as St Etienne completed a week that saw them book their first Stade de France final with victory at Brest.
Christophe Galtier's side beat Lille on penalties in midweek to book a Coupe de la Ligue final meeting with Rennes in Paris' famous stadium on April 20.
There was no hangover from the celebrations of that success tonight, however, as ASSE grabbed their first league win since November and moved back toward the European places.
For Brest it was a fourth successive home reverse and only their away form - they are unbeaten in six on their travels - has ensured they still hold a five-point buffer over the bottom three.
In the end they were unlucky not to snatch a point, with Eden Ben Basat's late drive deflected against a post, although they had been second best before Brandao's 50th-minute winner.
Sought-after striker Pierre-Emerick Aubameyang forced Brest goalkeeper Alexis Thebaux to punch clear his first-time shot after two minutes.
Aubameyang continued to threaten an early opener and while he beat Thebaux shortly after Johan Martial cleared off the line. The striker was then sent through on goal but Thebaux stood up well to make the save.
Ben Basat wasted Brest's best chance of the first half on 38 minutes when he dribbled past two defenders only to shoot into the side-netting.
St Etienne grabbed the decisive goal five minutes after half-time when Brandao ran on to Renaud Cohade's pass from the right. Thebaux raced off his line to apply pressure but the Brazilian kept his cool to roll the ball beyond him and into an empty net.
With 40 minutes left to hold out for a first league win in six St Etienne sat back.
Stephane Ruffier made a double save on the hour from Ben Basat before the goalkeeper got down well to keep out Charlison Benschop's low 22-yard shot.
Florian Lejeune headed a Bruno Grougi cross just over before Ruffier made his most important save, three minutes from the end, to deflect Ben Basat's well-hit 25-yarder on to a post.