Kia Sportage 2012: Shuddering brake problem

We are experiencing a problem with our 2012 Kia Sportage SI, which we purchased in February this year with less than 1000 km on the speedometer. On taking it for a test drive we noticed that when we braked there was a pronounced "clunking, shuddering" noise that appeared to come from the rear brakes and through the brake pedal. When we mentioned it to the salesman we were told it would disappear after a while because the vehicle was virtually brand new and had not been run in. The noise still persists, despite being taken back to the dealership three times. The service manager told us that the noise was caused by: 1. The car sitting on the dock in Melbourne too long and was due to surface rust on the brakes. 2. It was a normal car/running-in noise. 3. All Kia vehicles are known for this problem. 4. Driver technique. We are frustrated, annoyed and fed-up with the treatment and comments from the dealership. Could you please help us?