Emergency Braking and System Failure Response
What the exam tests
This domain tests what to do when pressure drops, a circuit fails, or the service brakes do not respond. The exam wants a safe sequence: recognize, stop, secure, inspect.
Low-pressure response
If the warning light and buzzer come on, pressure is below the safe range. Signal, move to the shoulder, stop, apply the parking brake, chock the wheels, and look for the cause. Do not try to drive to the next exit. As pressure falls further, the spring brakes will apply automatically.
Service-brake failure
If the pedal goes to the floor or a circuit fails, downshift, use engine braking, and apply the parking or emergency brake gradually. Look for an escape ramp or safe runoff area. Do not pump the brake pedal repeatedly in an air-brake system hoping to rebuild pressure.
Spring brakes will not release
Check system pressure first. If pressure is adequate, the problem may be a valve, inversion valve, relay valve, or chamber. Do not force the vehicle to move; get assistance.
Worked example
The buzzer sounds, the gauge reads 50 psi, and stopping distance increases. The correct action is to stop safely and inspect. Continuing because the brakes still feel partly effective is dangerous; the spring brakes may set unexpectedly.
Traps that cost marks
Do not ignore the warning until the springs set. Do not fan the pedal to build pressure. Do not use the spring brake as a routine slowdown. Do not continue after one circuit fails, even if the other circuit still works.