CDL Air Brake - Commercial Driver's License Air Brakes Parking Brake System Questions and Answers — Questions and Answers
Question 1: What provides the force to apply the parking brakes in a vehicle equipped with an air brake system?
- Air pressure from a dedicated parking brake tank.
- Hydraulic pressure from the master cylinder.
- Large, powerful springs located in the brake chambers. (Correct answer)
- The driver's physical effort applied to the brake pedal.
Correct answer: Large, powerful springs located in the brake chambers.
Parking brakes, also known as spring brakes, use the mechanical force of powerful springs to apply the brakes. Air pressure is used to hold these springs back (caged) during normal driving. When the driver pulls the parking brake control, air is released, and the springs expand to apply the brakes.
Question 2: A driver pulls the yellow, diamond-shaped knob on the dashboard to park the vehicle. What is happening within the air brake system?
- Air pressure is being sent to the brake chambers to apply the brakes.
- Air pressure is being released from the spring brake chambers, allowing the springs to apply the brakes. (Correct answer)
- The air compressor is being activated to build pressure specifically for the parking brakes.
- An electrical signal is sent to lock the service brakes in the applied position.
Correct answer: Air pressure is being released from the spring brake chambers, allowing the springs to apply the brakes.
The yellow, diamond-shaped knob is the parking brake control valve. Pulling this knob out releases the air pressure that holds the powerful parking brake springs in the 'caged' or released position. With the air pressure exhausted, the springs expand and mechanically apply the brakes.
Question 3: Which of the following conditions will cause the spring brakes to apply automatically?
- The service brake pedal is pressed and held for more than 60 seconds.
- The engine RPM drops below a certain level.
- Air pressure in the main system drops to a low level, typically in the 20-45 psi range. (Correct answer)
- The vehicle exceeds the posted speed limit while descending a steep grade.
Correct answer: Air pressure in the main system drops to a low level, typically in the 20-45 psi range.
Spring brakes are a critical safety feature. If there is a significant leak and the air pressure in the reservoirs drops to a critically low level (typically between 20 and 45 psi), the parking brake control valve will automatically pop out, releasing the remaining air pressure and causing the spring brakes to apply.
Question 4: Some vehicles are equipped with a control lever on the dash that allows the driver to apply the spring brakes gradually. What is this control called?
- An S-cam rotational controller.
- A slack adjuster override.
- An anti-lock braking actuator.
- A modulating control valve. (Correct answer)
Correct answer: A modulating control valve.
A modulating control valve allows the driver to control the application of the spring brakes, typically through a lever on the dash. This provides a 'feel' for the braking action and allows for a controlled stop if the service brakes fail.
Question 5: A driver is operating a combination vehicle and needs to park. What is the correct procedure for using the parking brakes?
- Apply only the trailer parking brakes to prevent stress on the tractor.
- Apply only the tractor parking brake and leave the trailer brakes released.
- Use the service brake pedal to hold the vehicle while shutting down.
- Apply the parking brakes for both the tractor and the trailer. (Correct answer)
Correct answer: Apply the parking brakes for both the tractor and the trailer.
When parking any vehicle, especially a combination vehicle, the parking brakes on all units should be applied to secure the entire vehicle safely. The yellow knob applies the tractor parking brakes, and the red knob (trailer air supply) applies the trailer emergency/parking brakes.
Question 6: A key safety rule states that a driver should never push the brake pedal down when the spring brakes are on. Why is this?
- It can cause the air compressor to overheat.
- It can damage brake components due to the combined force of the springs and air pressure. (Correct answer)
- It will immediately release the spring brakes, causing the vehicle to roll.
- It can drain the vehicle's battery by activating the stop lights.
Correct answer: It can damage brake components due to the combined force of the springs and air pressure.
Applying the service brakes (by pushing the pedal) while the spring brakes are already applied can create a dangerous level of force on the brake linkage, slack adjusters, and other foundation brake components. This combined force, known as compounding, can lead to damage or failure.
CDL Air Brake - Commercial Driver's License Air Brakes Parking Brake System Questions and Answers
What provides the force to apply the parking brakes in a vehicle equipped with an air brake system?