Free CDL Air Brake Parking and Emergency Brakes Questions and Answers — Questions and Answers
Question 1: What activates the parking brakes in an air brake system?
- Hydraulic pressure.
- Air pressure from the service system.
- Spring pressure when air is removed. (Correct answer)
- The foot brake pedal.
Correct answer: Spring pressure when air is removed.
In an air brake system, parking brakes are typically 'spring brakes' that are held in the released position by air pressure. When the driver activates the parking brake or if the air pressure drops too low (e.g., in a system failure), the air is released, allowing powerful springs to engage the brakes. This design ensures the vehicle will automatically brake if air pressure is lost.
Question 2: When should you use parking brakes?
- Only when parking on a hill.
- Every time you park the vehicle, unless brakes are hot or in freezing weather. (Correct answer)
- Only when parking in a flat area.
- During emergency braking situations.
Correct answer: Every time you park the vehicle, unless brakes are hot or in freezing weather.
Parking brakes should be used every time you park a vehicle to ensure it remains securely stationary. However, there are exceptions: if the brakes are extremely hot, engaging the parking brake can cause them to warp or fuse. In freezing weather, moisture can freeze the brake components, making them impossible to release, so wheel chocks should be used instead.
Question 3: What should you do if your parking brakes won’t release due to frozen brake components?
- Drive slowly until they release.
- Pour hot water over the frozen parts.
- Use a heater or other method to warm the brakes. (Correct answer)
- Use the emergency brake to override the parking brake.
Correct answer: Use a heater or other method to warm the brakes.
If parking brakes are frozen, attempting to force them to release can cause severe damage to the brake system. The safest and most effective method is to gently warm the frozen components, for example, by using a portable heater or waiting for warmer temperatures. This melts the ice and allows the brakes to release without damage.
Question 4: When do spring brakes automatically engage in a failure situation?
- When air pressure drops below 60 psi.
- When air pressure drops to 20-45 psi. (Correct answer)
- When the driver presses the brake pedal fully.
- When the ABS system is activated.
Correct answer: When air pressure drops to 20-45 psi.
Spring brakes are designed as an emergency braking system that automatically engages if the air pressure in the service brake system falls to a critically low level. This safety feature typically activates when the air pressure drops into the range of 20-45 psi. This ensures the vehicle will come to a stop even in the event of a major air leak or compressor failure, preventing a runaway vehicle.
Question 5: Can spring brakes and service brakes be applied simultaneously?
- Yes, to maximize braking force.
- No, it can damage the brake system. (Correct answer)
- Only in emergency braking situations.
- Only if the parking brake valve is manually released.
Correct answer: No, it can damage the brake system.
Applying both spring brakes (parking/emergency brakes) and service brakes simultaneously is generally not recommended and can be detrimental to the brake system. Spring brakes are for parking or emergency situations when service brakes fail, and their combined application can put excessive stress on components. This can lead to damage or uneven braking, which is why they should not be used together.
Free CDL Air Brake Parking and Emergency Brakes Questions and Answers
What activates the parking brakes in an air brake system?