CDL Air Brakes Practice Test Part 2

1. Which statement accurately describes the stopping distance difference between a loaded and an empty commercial motor vehicle with air brakes?

2. How much time does air brake lag typically add between pressing the brake pedal and the brakes beginning to engage?

3. What is the primary mechanical difference between wedge brakes and S-cam brakes in an air brake foundation brake system?

4. What is the key difference between manual slack adjusters and automatic slack adjusters on an air brake system?

5. Why must drivers manually drain air tanks regularly, even on vehicles equipped with automatic drain valves?

6. What is the function of an alcohol evaporator in an air brake system, and when should it be used?

7. A driver is performing an air brake system check during a pre-trip inspection. The driver pumps the brake pedal with the engine off until the air pressure drops to 85 PSI, then starts the engine and idles. Within 45 seconds, the pressure rises from 85 PSI to 100 PSI and the compressor continues building. What does this observation indicate about the governor and compressor?

8. A driver is traveling at 60 MPH on a highway when a hazard appears ahead. The driver immediately presses the brake pedal. Given that air brake lag is 1/2 second minimum, approximately how many feet does the vehicle travel before the brakes actually begin to engage due to this lag?

9. A driver operates the same tractor-semitrailer on two different days: Day 1 with a 45,000-pound load on dry pavement, and Day 2 empty (no cargo) on the same dry pavement at the same speed. During brake applications on Day 2, the driver notices the brakes seem less effective and the vehicle slides more easily. Which analysis explains this observation and what adjustment should the driver make?

10. A driver observes the following during an extended pre-trip air brake test: with the engine running, the compressor builds pressure to exactly 125 PSI and stops; the driver applies brakes several times, dropping pressure to 95 PSI; the compressor resumes and builds to 125 PSI; the driver continues applying brakes until pressure drops to 55 PSI, at which point the low-pressure warning buzzer activates. Which component or setting should the driver question based on these observations?

11. How do spring brakes (parking/emergency brakes) in an air brake system function differently from service brakes?

12. What is the function of the safety valve in an air brake system, and at approximately what pressure does it typically release?

13. What is the purpose of the tractor protection valve in a combination vehicle air brake system?

14. A driver is inspecting a CMV’s foundation brakes and notices a wedge-type brake system rather than the common S-cam type. How does the wedge brake mechanism apply braking force differently from an S-cam brake?

15. A driver is preparing to descend a steep mountain grade with a loaded tractor-semitrailer. Another driver suggests using the trailer hand valve (trolley valve) continuously during the descent to help control speed. What is the correct response to this suggestion?

16. A driver notices the air pressure gauge reading has dropped from 125 PSI to 35 PSI while driving, and the vehicle has suddenly slowed as the spring brakes have partially applied automatically. What is the correct immediate action?

17. What is the correct procedure for draining air tanks during vehicle maintenance, and why is this procedure important for air brake system safety?

18. What determines the braking force produced by a brake chamber in an air brake system, and how does chamber size affect this force?

19. A driver operates a fully loaded Class A tractor-semitrailer on a 6% downgrade lasting 4 miles. The driver maintains speed by applying service brakes frequently and riding the brakes for most of the descent. Near the bottom of the grade, the brakes begin to fade, feel soft, and provide reduced stopping power. The driver then applies maximum brake pressure, which causes complete brake failure. What two critical errors caused this outcome?

20. Two identical Class A tractor-semitrailer vehicles are traveling at 55 MPH on a dry road. Vehicle A is fully loaded at 80,000 lbs. Vehicle B is empty at 35,000 lbs. Both drivers apply brakes simultaneously with equal force. Vehicle B takes noticeably longer to stop than Vehicle A, and the driver of Vehicle B must increase brake pressure to stop effectively. What explains this counterintuitive result, and what does it indicate about brake force requirements?