CDL Air Brake - Commercial Driver's License Air Brakes Using Service and Emergency Brakes Questions and Answers — Questions and Answers
Question 1: When making an emergency stop with a vehicle that has air brakes, what is the 'stab braking' method?
- Applying the brakes as hard as possible without locking the wheels.
- Pumping the brake pedal quickly and lightly to prevent overheating.
- Applying the brakes fully until the wheels lock, then releasing them until the wheels start rolling and reapplying. (Correct answer)
- Alternately applying the service brake and the parking brake to bring the vehicle to a stop.
Correct answer: Applying the brakes fully until the wheels lock, then releasing them until the wheels start rolling and reapplying.
Stab braking involves applying the brakes all the way to lock the wheels, then releasing them as soon as they start rolling again, and then reapplying the brakes fully. This technique is used for emergency stops in vehicles without anti-lock brakes (ABS).
Question 2: You are driving a vehicle with a dual air brake system. The low air pressure warning light and buzzer come on. What is the safest course of action?
- Continue driving to the nearest service station while monitoring the gauges.
- Downshift and use the engine brake to slow down, preserving the remaining air pressure.
- Increase your following distance and avoid using the brakes unless absolutely necessary.
- Stop and safely park the vehicle as soon as possible. (Correct answer)
Correct answer: Stop and safely park the vehicle as soon as possible.
If the low air pressure warning activates, it indicates a potential air leak or system failure. The safest action is to pull over and stop immediately while you still have enough air pressure to control the service brakes. Continuing to drive is dangerous as the spring brakes could engage automatically when pressure drops too low, causing a sudden and uncontrolled stop.
Question 3: What is 'brake lag' in a vehicle with air brakes?
- The distance the vehicle travels after the driver decides to brake but before they physically apply the pedal.
- The time it takes for the air to flow through the lines and apply the brakes after the pedal is pushed. (Correct answer)
- The fading of brakes due to excessive heat from overuse on a downgrade.
- The delay caused by wet brake linings, which reduces braking efficiency.
Correct answer: The time it takes for the air to flow through the lines and apply the brakes after the pedal is pushed.
Brake lag is the time delay (often half a second or more) from when the brake pedal is pressed to when the compressed air travels through the system to activate the brakes. This lag adds to the vehicle's total stopping distance.
Question 4: A driver is descending a long, steep grade. Which of the following is the correct method for using the service brakes?
- Apply light, steady pressure on the brake pedal all the way down the grade.
- When speed increases to the 'safe speed', apply the brakes firmly to reduce speed to approximately 5 mph below the safe speed, then release. (Correct answer)
- Pump the brakes on and off every few seconds to keep them cool.
- Use the hand-operated trailer brake exclusively to save the tractor's service brakes.
Correct answer: When speed increases to the 'safe speed', apply the brakes firmly to reduce speed to approximately 5 mph below the safe speed, then release.
The correct technique is to go down the grade in a low gear, and when your speed reaches your designated 'safe speed', apply the brakes firmly to slow down to about 5 mph below that speed, then release. This method prevents the brakes from overheating and fading. Light, steady pressure will cause excessive heat and potential brake failure.
Question 5: The emergency brake system in a vehicle with air brakes uses parts of which two other systems?
- The service brake and the engine retarder systems.
- The parking brake and the hydraulic brake systems.
- The service brake and the parking brake systems. (Correct answer)
- The anti-lock brake and the traction control systems.
Correct answer: The service brake and the parking brake systems.
The emergency brake system is designed to use parts of both the service and parking brake systems to stop the vehicle in the event of a brake system failure. For example, a loss of air pressure will cause the powerful spring brakes (which are also used for parking) to apply.
Question 6: During a pre-trip inspection, you notice that the spring brakes do not automatically apply when the air pressure drops to the 20-45 PSI range. This indicates a problem with the:
- Air compressor governor.
- Brake drum temperature.
- Service brake pedal.
- Parking brake control valve. (Correct answer)
Correct answer: Parking brake control valve.
The parking brake control valve (often a yellow, diamond-shaped knob) is designed to pop out, causing the spring brakes to apply, when air pressure drops to a range between 20 and 45 PSI. If this doesn't happen, the valve may be faulty, creating a serious safety issue.
CDL Air Brake - Commercial Driver's License Air Brakes Using Service and Emergency Brakes Questions and Answers
When making an emergency stop with a vehicle that has air brakes, what is the 'stab braking' method?