Brake Master Cylinder problems of the 2000 Chevrolet C/k 3500

First time the brake failure occurred was in the driveway. My son, a newly licensed driver was taking the vehicle to school when he came back into the house and said the brakes on the truck don't feel right and the pedal goes to the floor. He stated that as he was backing out of the driveway, the brake pedal went to the floor. As a mechanic, I took the vehicle to the shop and found the brake line from the master cylinder to the rear brakes had failed. I replaced this line. Less than a month later another brake line burst when I had my foot on the brake in a parking lot. This time it was the brake line from the tee near the differential going to the left rear brake. All brake lines will now be replaced. My personal opinion is this is a defective part. I know I live in an area where deicing chemicals are used on the roads in winter, however, to avoid rust problems on this vehicle, the underside of this vehicle is power washed weekly during the winter months to remove these chemicals. Frankly, the brake lines do not look that bad. It's like they just develop tiny "pin holes" in specific areas.