CDL Air Brakes TestAir Brake System PartsEasy
A driver performs a pre-trip inspection and observes that the parking brake control knob pops out automatically when the air pressure in the system drops below 40 psi. What does this indicate?
- AA leak in the primary air tank.
- BA faulty low air pressure warning switch.
- CA malfunction of the air compressor governor.
- DNormal operation of the spring brakes and parking brake system.
Show answer & explanationAnswer & explanation
Correct answer: D. Normal operation of the spring brakes and parking brake system.
Spring brakes (parking/emergency brakes) are designed to apply automatically when the air pressure in the system falls to a dangerously low level, typically between 20-45 psi. This is a safety feature to ensure the vehicle stops if air pressure is lost.
Why the other options are wrong
- A. While a leak in the primary tank could cause the pressure drop, the question asks what the automatic pop-out indicates, which is a normal response to low pressure.
- B. A faulty warning switch would affect the light/buzzer, not the physical application of the parking brake.
- C. The governor controls compressor cycling, not automatic parking brake application due to low pressure.
Spring Brake Automatic Engagement
The safety feature where spring brakes automatically apply when the air pressure in the air brake system drops below a specified threshold (typically 20-45 psi), acting as an emergency brake.
- Prevents uncontrolled vehicle movement during air loss.
- Indicated by the parking brake control knob popping out.
- A crucial safety mechanism in air brake systems.
- Requires sufficient air pressure to release.
Memory trick: Pressure low, knob will go, safety's on, a crucial show.