Commercial Driving Air Brakes — Questions and Answers
Question 1: What is the purpose of the air compressor in an air brake system?
- To cool the brakes
- To pump air into storage tanks to provide the energy for braking (Correct answer)
- To inflate the tires
- To power the horn
Correct answer: To pump air into storage tanks to provide the energy for braking
The air compressor pumps air into storage tanks, building the air pressure that is used as the force to apply the brakes when the driver presses the brake pedal.
Question 2: At what PSI should the low air pressure warning activate?
- 40 PSI
- 60 PSI (Correct answer)
- 80 PSI
- 100 PSI
Correct answer: 60 PSI
The low air pressure warning signal must activate before air pressure drops below 60 PSI, alerting the driver that the air brake system is losing pressure.
Question 3: What happens when the air pressure drops too low in the brake system?
- The brakes release completely
- The spring brakes automatically apply, bringing the vehicle to a stop (Correct answer)
- The engine shuts off
- Nothing, the brakes work the same
Correct answer: The spring brakes automatically apply, bringing the vehicle to a stop
Spring brakes are held off by air pressure during normal operation. When pressure drops below about 20-45 PSI, the springs apply the brakes automatically as a safety measure.
Question 4: What is the purpose of the air brake system's governor?
- To control the speed of the vehicle
- To control the air compressor by starting it when pressure drops and stopping it when pressure reaches the maximum (Correct answer)
- To regulate brake pedal pressure
- To measure tire pressure
Correct answer: To control the air compressor by starting it when pressure drops and stopping it when pressure reaches the maximum
The governor controls the air compressor, engaging it when tank pressure drops to about 100 PSI and disengaging it when pressure reaches about 125 PSI.
Question 5: What is 'brake lag' in air brake systems?
- A braking defect
- The delay between pressing the brake pedal and the brakes actually applying, typically about 0.5 seconds (Correct answer)
- The sound brakes make when applied
- The distance traveled while braking
Correct answer: The delay between pressing the brake pedal and the brakes actually applying, typically about 0.5 seconds
Air brake lag is the time it takes for air to travel through the lines to the brake chambers after the pedal is pressed. This adds to stopping distance compared to hydraulic brakes.
Question 6: What is the air brake cut-in and cut-out pressure range?
- 50-75 PSI
- 80-100 PSI
- 100-125 PSI (Correct answer)
- 125-150 PSI
Correct answer: 100-125 PSI
The governor typically cuts in the compressor at about 100 PSI and cuts it out at about 125 PSI, maintaining system pressure within this operating range.
Commercial Driving Air Brakes
What is the purpose of the air compressor in an air brake system?