Graded Downgrade With Brake Fade
You are driving a 34,000-lb gross straight truck with air brakes down a 6% grade that runs 5 miles. At the top, the posted truck speed is 35 mph; you are at 40 mph in 4th gear of a 6-speed manual, engine brake on, air pressure 110 psi. The low-pressure warning is set at 60 psi. About halfway down, the service brake pedal feels soft and the truck accelerates to 48 mph despite steady pressure; rpm is near the governed limit. The air gauge has fallen to 92 psi and is moving down slowly. You smell hot brake lining. There is a paved runaway truck ramp 1/3 mile ahead on the right, and a narrow shoulder beyond it. Traffic is light behind you. You have no trailer.
1. What is the most likely cause of the soft pedal and rising speed, and what should you do in the next few seconds?
Read the worked answer
The service brakes are fading because they are overheated from continuous use on a long grade; the soft pedal and hot-lining smell fit that, and the engine brake alone is not holding 34,000 lb at 6%. Do not stay in 4th or coast in neutral. Downshift to 3rd, let the engine brake work, and use the service brakes in short, firm snubs rather than light continuous pressure. Aim for the runaway ramp if speed is still climbing or you cannot get a lower gear. Do not use the parking brake at speed. Keep the steering tires rolling and the truck pointed at the safest escape path.
2. If the low-pressure warning sounds at 60 psi while you are still on the grade, how does that change your plan?
Read the worked answer
A 60-psi warning means air pressure is near the point where spring brakes can apply automatically, so continuing at normal speed is not an option. Use the engine brake and downshift, but make the runaway ramp or the first safe pullout the priority. If the service brakes are already weak, do not wait for the spring brakes to set on their own; they can lock the rear wheels and cause a skid. After stopping, set the parking brake, chock if needed, and do not move until the air system is repaired and pressure holds. If you cannot stop before the ramp, use the ramp.
3. Why is applying the parking brake at 45 mph a poor immediate response?
Read the worked answer
In an air-brake truck, the parking brake is spring-applied and typically acts on the rear wheels. Pulling it at 45 mph can lock those wheels abruptly, causing a jackknife or loss of steering control, especially on a downgrade. It also does not give the controlled, progressive retardation needed to shed speed. The parking brake is for holding a stopped vehicle, not for stopping a moving one; use downshifting, engine braking, snubs, and the escape ramp instead.