CDL Air Brake - Commercial Driver's License Air Brakes Using Service and Emergency Brakes Questions and Answers

Question 1

When making an emergency stop with a vehicle that has air brakes, what is the 'stab braking' method?

Accepted Answer

Applying the brakes fully until the wheels lock, then releasing them until the wheels start rolling and reapplying.

Answer

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?

Accepted Answer

Stop and safely park the vehicle as soon as possible.

Answer

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?

Accepted Answer

The time it takes for the air to flow through the lines and apply the brakes after the pedal is pushed.

Answer

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?

Accepted 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.

Answer

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?

Accepted Answer

The service brake and the parking brake systems.

Answer

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.

CDL Air Brake - Commercial Driver's License Air Brakes Test Practice Test

CDL Air Brake - Commercial Driver's License Air Brakes Test Practice Test

CDL Air Brake - Commercial Driver's License Air Brakes Using Service and Emergency Brakes Questions and Answers