Safe Driving Practices

Study questions with answers

12 sample Safe Driving Practices questions with the correct answer, a short explanation, and the official handbook reference. Read through them, then take the quiz above.

1. 1. What does an alcohol evaporator do in some air brake systems?

   Correct answer: It lowers the chance of ice forming in the brake valves during cold weather

   By putting alcohol into the air, the evaporator lowers the risk of ice forming in valves and other parts during cold weather; daily tank draining is still required.

   Source: South Carolina CDL Manual — Section 5.1.5: Alcohol Evaporator

2. 2. How much pressure should you let build in both the primary and secondary systems before setting off in a dual-air-system vehicle?

   Correct answer: At least 100 psi

   Give the compressor time to reach a minimum of 100 psi in both the primary and secondary systems before you drive.

   Source: South Carolina CDL Manual — Section 5.2: Dual Air Brake

3. 3. When testing the service brakes, at what speed and how should you apply them?

   Correct answer: Creep forward at roughly five mph, then press the pedal firmly

   Release the parking brake, move ahead slowly at about five mph, and press the pedal firmly, watching for pulling to one side, an odd feel, or delayed stopping.

   Source: South Carolina CDL Manual — Section 5.3.3: Final Air Brake Check

4. 4. Where is the safety relief valve located, and at about what pressure is it usually set to open?

   Correct answer: In the first tank the compressor fills, opening at about 150 psi

   The safety valve sits in the first reservoir the compressor pumps into and typically opens near 150 psi to protect the system from over-pressure; if it vents air, a mechanic should look at it.

   Source: South Carolina CDL Manual — Section 5.1.6: Safety Valve

5. 5. At roughly what tank pressure does the governor tell the compressor to stop pumping (the cut-out level)?

   Correct answer: About 125 psi

   The governor cuts the compressor out near 125 psi; when pressure later falls to the cut-in level around 100 psi, it lets the compressor start again.

   Source: South Carolina CDL Manual — Section 5.1.2: Air Compressor Governor

6. 6. When inspecting brake drums, a crack is unacceptable once it is longer than what?

   Correct answer: Half the width of the friction area

   A drum or disc is unacceptable once a crack runs more than half the width of its friction area; linings can't be loose, grease-soaked, or too thin.

   Source: South Carolina CDL Manual — Section 5.3.2: Walk-around Inspection

7. 7. Which type of foundation brake is the most common on air brake vehicles?

   Correct answer: The s-cam drum brake

   The s-cam drum brake is the most common foundation brake; wedge and disc types are found less often.

   Source: South Carolina CDL Manual — Section 5.1.8: Foundation Brakes

8. 8. Air brakes are often described as three braking systems working together. Which three make up that combination?

   Correct answer: Service, parking, and emergency systems

   An air brake setup combines three systems: the service brakes you work with the pedal, the parking brakes you set with a hand control, and the emergency brakes that borrow parts of the other two if something fails.

   Source: South Carolina CDL Manual — Section 5: Air Brakes

9. 9. Why is it a bad idea to press and release the brake pedal repeatedly for no reason?

   Correct answer: It can empty the tanks faster than the compressor can refill them

   Each release lets some air escape, so needless fanning of the pedal can drain the tanks faster than the compressor can refill them; if pressure drops too low, the brakes stop working.

   Source: South Carolina CDL Manual — Section 5.1.7: The Brake Pedal

10. 10. How is a manually operated air tank drain valve worked?

    Correct answer: By turning it a quarter turn or pulling a cable

    A manual drain is opened with a quarter turn or by pulling a control cable, and the driver must do it themselves; automatic drains expel the water and oil on their own.

    Source: South Carolina CDL Manual — Section 5.1.4: Air Tank Drains

11. 11. What does an antilock braking system (ABS) actually do?

    Correct answer: It prevents the wheels from locking during a hard stop

    ABS is a computerized system that prevents wheel lockup during hard stops; it activates only when a wheel is about to lock.

    Source: South Carolina CDL Manual — Section 5.1.16: Antilock Braking Systems (ABS)

12. 12. Why must the emergency and parking brakes be held on by mechanical force rather than by air pressure alone?

    Correct answer: Because air pressure can eventually leak away

    Air can slowly bleed off over time, so parking and emergency brakes rely on mechanical force — usually powerful springs — to stay applied.

    Source: South Carolina CDL Manual — Section 5.1.14: Spring Brakes