brake hard to push

My car is an 07 Aveo, with 8000 miles on it, automatic. I notice that when the engine is cold, the brake is hard to push when the car is the reverse shifting. Plus, the car doesn't stop when brake is applied. In addition, the brake groans when I slowly release the brake paddle. I took it to a dealership (since the car is still under warranty). They told me that the vacuum line needs to be replaced and they did. However, the problem is still there. Please tell me what else possibly be the cause of it.