Q: brake line ruptures while driving in city

While I was coming to a red light at an intersection at about 20mph, I applied the brakes. As I found out later, the rear brake line ruptured because of rust. At the time it ruptured, I was able to safely stop my truck. I am so fortunate that this did not happen on the highway. My truck also has extensive body and frame surface rust. I mostly park outside and live in the mid atlantic regions. It has very low mileage because I do not drive it very much. The manufacturer will not help me with the problem saying it is an environmental cause. Since I don’t drive the truck much at all, I don't understand why it would rust so much. Is there some way that I can treat this rust to inhibit it from increasing?