Emergency Braking and Brake Failure Response
This domain tests emergency braking and brake failure response: low pressure, partial circuit failure, spring brake application, fade, and runaway situations.
Failure response
If the low-pressure warning sounds, stop safely as soon as possible; do not try to reach a repair shop. If service brakes fail, downshift, use engine braking, and apply the parking or spring brakes gradually to avoid skidding. Look for an escape ramp or safe pull-off and warn other traffic. If one circuit fails, the other may still provide braking; use it to stop. If brakes fade from heat, pull over and let them cool rather than continuing. Spring brakes apply automatically at about 20–45 psi and will stop the vehicle, but steering control still matters.
Worked example
A warning buzzer sounds and the gauge falls below 60 psi on a downgrade. The correct action is to brake with remaining pressure, downshift, move to the shoulder, and stop. Continuing to the next exit is wrong because pressure may fall to the spring brake range and the vehicle may stop where control is poor. If pressure reaches 20–45 psi, the spring brakes set; hold the steering wheel and let them bring the vehicle to a controlled stop.
Traps that cost marks
Do not ignore the warning and keep driving. Do not pump the air brake pedal expecting pressure to rebuild. Do not use the trailer hand valve as a primary emergency brake. Do not pass an escape ramp when brakes are fading. Do not release the parking brake after a low-pressure stop without finding and fixing the leak.