Emergency Braking and Brake Failure Response
This domain tests what to do when service brakes fail, pressure drops, or a trailer separates. The exam wants a controlled response, not a panic stop that causes a skid.
Low-pressure warning
If the buzzer and light activate, stop as soon as it is safe. Do not try to reach the destination. Pressure loss can lead to automatic spring brake application at 20–45 psi, possibly in traffic.
Service brake failure
- Keep steering and look for an escape route or runaway truck ramp.
- Downshift and use engine braking.
- Apply the parking brake gradually. Full spring brake application at highway speed can lock the wheels and cause a skid.
- If a trailer breaks away or its supply line separates, the tractor protection valve and trailer spring brakes should apply the trailer brakes.
Brake fade or fire
If brakes fade from heat, stop and let them cool. Do not continue down the grade relying on hot brakes.
Worked example
On a highway, pressure falls below 60 psi and the warning sounds. The driver signals, exits at the next safe point, downshifts, and applies the parking brake gradually after slowing. Continuing would risk spring brakes applying at speed and a loss of control.
Traps
- Slamming the parking brake control at highway speed.
- Continuing after the low-pressure warning.
- Thinking an engine-off roll preserves air pressure; leaks continue.
- Forgetting that a separated trailer’s spring brakes should apply automatically.
Key points
- Stop safely when the low-pressure warning activates.
- Use engine braking and downshifting after service brake failure.
- Apply the parking/spring brake gradually, not suddenly at speed.
- Trailer separation should apply trailer spring brakes through the protection valve.
- Stop and cool overheated brakes instead of continuing.