Brake Light On problems of the 2006 Cadillac STS

Car (2006 Cadillac Sts) was being driven, three occupants in vehicle, when brakes were applied. Brake pedal became soft and went to the floor upon braking for an intersection. Through pumping the brakes, car was able to be stopped. The brake warning light came on. Car was limped home at approximately 15 mph. Tow service was called and car was taken to dealership from which it was purchased (a non Cadillac dealership). That dealership checked the issue and stated that all four metal brake lines were severely corroded. The car was then sent to the local Cadillac dealership, which also stated that all four metal brake lines had corroded to the point to where they were all leaking. The background on the car is that we are the second owner. It was purchased seven months ago (Feb. 2012) from the used car inventory of a local Toyota dealership. The car was a local trade-in. The area that the car has been in since new is the peidmont area of south carolina, 70 miles from the coast, an area with mild winters (rare to get snow or ice). Concern is the complete, simultaneous failure of all four metal brake lines in a car that is less than 6 years old, which has resided in a mild climate. As of this writing, the brake lines have not been repaired/replaced; currently trying to work to resolve this issue with all parties involved.