Loss most of my brakes on my 94 dually a few days ago. I was in heavy traffic and when I applied the brakes, at 40 MPH, the brake light located to the right of the steering wheel on the dash lit up and the pedal suddenly became hard,. . much like it does if you use the brakes too much when you back up, especially with an automatic. I really had to lay on the brakes to stop as it was like stopping a car with out the engine running. I think that I am lucky that I wasn't running loaded at the time. Thought that my vacuum pump or my brake booster had gone out, but it turned out to be the vacuum line that goes into the cab on the driver's side had broken. Checked the rest of the plastic lines and found some that were brittle. Hard to believe that such a small leak could affect the braking system so much. I think later models,. . those after 94,. . develop more vaccuum, but I don't know if they can withstand a broken line without the same effects.