Downhill Grade with Fading Brakes
You are descending a long 6% grade in a loaded tractor-trailer at 55 mph. Gross weight is 76,000 lbs. You have been using the service brakes to hold speed because the engine brake is set on its lowest setting and the terrain is curvy. About two-thirds of the way down, you notice you need to push the pedal harder and further to get the same slowing, and you can smell hot brakes. Your speed has crept up to 62 mph. The air pressure gauges still read 110 psi on both circuits.
1. What is happening, and what is the correct immediate response?
Read the worked answer
This is brake fade - the drums and linings are overheating, and friction is dropping, so you need more pedal for less stopping. The gauges reading 110 psi tells you this is not an air supply problem; it is a heat problem. The correct response is to stop using the service brakes to control speed and get the vehicle slowed and stopped safely: downshift to let engine braking and gearing hold you back, and find a safe place to pull over and let the brakes cool. Do not keep riding the pedal harder - that adds more heat and makes fade worse. Do not use the parking brake to stop a moving vehicle at speed as a substitute, and never fan or pump the brakes on a grade hoping to cool them; that just cycles more heat through them.
2. Why is the 110 psi reading not reassuring in this situation, and what would make it a genuine emergency?
Read the worked answer
Air pressure and brake friction are separate systems. The gauge confirms the compressor and reservoirs are supplying air, but fade happens at the drum/linings where heat has reduced friction - plenty of air pressure pushing on a faded brake still gives you weak stopping. It becomes a genuine emergency if pressure starts falling (a leak or a compressor that can't keep up), if the pedal goes to the floor, or if you cannot hold speed even with gearing and engine braking. In that case you use the escape ramp or the least hazardous runoff, and you do not ride it out hoping the grade ends.
3. After you stop and cool down, what should you check before continuing, and how should you drive the rest of the grade?
Read the worked answer
Walk around and check for dragging brakes (hot drums on one side only can mean a stuck slack adjuster or a brake that isn't releasing), look for cracked drums or glazed linings, and confirm the pushrods return fully. Check that the slack adjusters are within adjustment. Before rolling, confirm both gauges are at normal operating pressure and do a quick applied-brake leak check. For the rest of the grade, select a lower gear before you start down so engine braking carries the load, use the service brakes in short controlled applications rather than continuous light pressure, and keep speed low enough that you rarely need the pedal. If the fade returns, stop again rather than pushing through it.