One problem related to brake drum wheel cylinder has been reported for the 2009 Toyota Prius. The most recently reported issues are listed below.
We rented a 2009 Prius when we were in las vegas. We almost got into a collision in the parking lot of a casino when someone pulled out in front of me all of a sudden. I pressed on the brake pedal as hard as a could and the car did not want to stop. I t was if the pedal was pushing on air. It did not go down as expected and the car barely came to a stop. This happened well before all the recalls with gas pedals sticking and such. I think that the sudden application of the brake pedal might of locked up the cylinder. The effect was similar to trying to push on a shock absorber really fast. They lock up and don't move very much. I am no small person 6'-2" and 285 lbs. And I was stomping on it !.