CDL Air Brakes TestAir Brake System PartsHard
A commercial vehicle is equipped with an Anti-lock Braking System (ABS). During an emergency stop on a slippery surface, the driver feels a pulsating sensation in the brake pedal. What does this sensation indicate?
- ANormal operation of the ABS system.
- BA major leak in the service brake line.
- CThe slack adjusters are improperly adjusted.
- DThe brake drums are severely out of round.
Show answer & explanationAnswer & explanation
Correct answer: A. Normal operation of the ABS system.
A pulsating sensation in the brake pedal during hard braking, especially on slippery surfaces, is a characteristic and normal indication that the ABS system is actively modulating brake pressure to prevent wheel lock-up. This is how the driver feels the system working.
Why the other options are wrong
- B. A major leak in the service brake line would lead to a loss of brake pressure and a 'soft' or 'sinking' pedal, not a pulsating sensation.
- C. Improperly adjusted slack adjusters would cause excessive pedal travel before brake engagement, not a pulsating sensation during braking.
- D. Severely out-of-round brake drums would cause a pulsating or vibrating feeling in the pedal even during normal, light braking, not specifically during an emergency stop where ABS would activate.
ABS Pedal Pulsation
During active anti-lock braking, a pulsating sensation felt in the brake pedal is a normal indication that the ABS system is rapidly applying and releasing brake pressure to prevent wheel lock-up.
- Normal and expected ABS function.
- Allows driver to maintain steering control.
- Occurs during hard braking or on slippery surfaces.
Memory trick: Pedal pulses hard and fast, ABS is working, you'll be safe at last!