CDL Air Brakes TestUsing Air BrakesEasy
A driver needs to make an emergency stop in a commercial vehicle equipped with air brakes and Anti-lock Braking System (ABS). What is the correct procedure for braking with ABS in an emergency?
- APump the brake pedal rapidly to prevent wheel lock-up.
- BEngage the parking brake momentarily to reduce speed before applying the service brakes.
- CApply full, firm pressure to the brake pedal and hold it down until the vehicle stops.
- DApply moderate pressure to the brake pedal, then release and reapply if wheels begin to lock.
Show answer & explanationAnswer & explanation
Correct answer: C. Apply full, firm pressure to the brake pedal and hold it down until the vehicle stops.
When an air-braked vehicle has ABS, the correct emergency braking procedure is to apply full, firm pressure to the brake pedal and hold it down. The ABS system will automatically modulate the brake pressure to prevent wheel lock-up, allowing the driver to steer.
Why the other options are wrong
- A. Pumping the brakes is the wrong technique for ABS and can defeat its purpose, as it prevents the system from sensing wheel lock-up correctly.
- B. The parking brake should never be used to slow a moving vehicle, especially in an emergency, as it can cause loss of control.
- D. This describes a form of 'controlled braking' or 'stab braking' which is used without ABS, or if ABS fails, but not with a working ABS.
Emergency Braking with ABS
The technique for stopping an air-braked vehicle with ABS in an emergency, involving applying and holding full pressure on the brake pedal.
- Apply full, firm pedal pressure.
- Hold pedal down, do not pump.
- ABS automatically prevents wheel lock-up.
- Allows steering control during braking.
Memory trick: ABS: Stomp and Steer, no fear!