Emergency Braking, Brake Fade, and Malfunctions
Scope
Emergency Braking, Brake Fade, and Malfunctions tests your response to low pressure, circuit failure, overheated brakes, and a brake that will not release or apply correctly. Questions focus on recognizing the problem and choosing the safest action.
Core rules
- If the low-pressure warning sounds, stop safely. Spring brakes will apply as pressure falls to about 20–45 psi; do not wait for that to happen in traffic.
- If service brakes fail, downshift, use engine braking, warn others, and look for an escape ramp or safe turnout. Use the parking brake only as a last resort, gradually, because it can lock the rear wheels.
- Brake fade is caused by heat, not by an air leak. The pedal may feel firm while stopping power drops. Shift to a lower gear, use engine braking, and stop to cool the brakes.
- Do not fan or pump overheated brakes; that adds heat and makes fade worse.
- If one air circuit fails, stop. The remaining circuit may get you to a safe place, but not to the end of the route.
- A dragging brake, stuck slack adjuster, or compressor failure is an out-of-service condition until repaired.
Worked example
On a long downgrade the driver smells hot brakes and sees weak braking, but the pedal is firm and the low-pressure warning is off. That points to fade, not air loss. The correct response is to downshift, use engine braking, and stop to cool. Pumping the pedal would add heat and reduce braking further.
Traps
Do not confuse fade with low pressure; the warning and gauge tell the difference. Do not use the trailer hand valve for an emergency stop; it can cause a jackknife. Do not ignore a circuit failure because the vehicle still slows.