Emergency Braking and Brake Failure Response
Emergency Braking and Brake Failure Response
This domain tests what to do when service brakes fail, pressure drops, or a stop is unavoidable. The exam rewards controlled, progressive actions and penalizes panic moves that lock wheels or cause a jackknife.
Core rules
- If the low-pressure warning sounds, stop safely before spring brakes apply. Move to the shoulder, use flashers, and set the parking brake once stopped.
- If service brakes fail, downshift through the gears to use engine braking, then apply the parking/spring brake gradually. Do not slam the parking brake at highway speed; a sudden lockup can cause a skid or loss of control.
- On a downgrade with brake failure, look for an escape ramp, a runaway truck ramp, or a safe off-road area. Use the ramp rather than continue at increasing speed.
- If a trailer breaks away or an air line separates, the tractor protection valve should close and the trailer spring brakes should apply. Do not try to reconnect a pressurized line by hand.
- After any emergency stop, do not drive until the cause is found and the system builds and holds pressure.
Worked example
A driver's service brake pedal goes to the floor on a mountain grade. The low-air warning is on and the truck is accelerating. The driver downshifts, steers for the escape ramp, and applies the parking brake progressively as the ramp slows the truck. The reasoning: engine braking and the ramp reduce speed before the spring brakes lock the wheels; slamming the parking brake at 60 mph would likely cause a skid.
Traps
Do not shut off the engine to “save air”; you need engine braking and steering. Do not pump a failed air brake system. Do not assume the spring brakes will stop you gently-they apply hard at low pressure.