Emergency Braking and Brake Failure Procedures
The exam tests what you do when braking is degraded or gone: which controls to use, in what order, and what not to do. Answers are judged on the safe sequence, not just the final action.
If the service brakes fail
- Downshift to use engine compression and gearing to slow the vehicle.
- Apply the parking (spring) brake gradually - a hard, sudden application can lock wheels and cause a skid.
- Look for an escape ramp, then for a side road, field, or upgrade.
- As a last resort, rub the guardrail or curb to scrub speed, or steer for something that will give way rather than a solid object.
- Sound the horn and warn traffic.
If air pressure is lost
- The spring brakes apply automatically at roughly 20–45 psi and will stop the vehicle. Do not fight them; steer to the shoulder while they set.
- On a combination, the tractor protection valve closes first and sets the trailer brakes, which helps straighten the rig.
If a trailer breaks away
- The separated emergency (supply) line exhausts, triggering the tractor protection valve and the trailer's spring brakes.
Emergency braking technique
- Stab braking (apply fully, release when wheels lock, reapply when rolling) is for emergency stops only.
- Controlled braking is preferred when you must stop and stay in lane.
Worked example
The pedal drops to the floor with no resistance and the low pressure warning is on. Reasoning: this is an air loss, not a fade or adjustment problem. The spring brakes will set shortly in the 20–45 psi range; the driver downshifts, steers to the shoulder, and lets them stop the unit rather than pumping a dead pedal.
Traps that cost marks
Pumping the pedal instead of downshifting; applying the spring brake all at once at highway speed; assuming a torn supply line leaves the trailer without brakes.