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. The governor lets the compressor start pumping again once tank pressure falls to what approximate level (the cut-in point)?

   Correct answer: About 100 psi

   Once pressure in the tanks drops to around 100 psi, the governor's cut-in setting allows the compressor to resume pumping air.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.2 Air Compressor Governor

2. 2. At about what pressure is the system's safety relief valve usually set to open?

   Correct answer: About 150 psi

   The safety valve, mounted in the first tank the compressor feeds, guards against excess pressure and is normally set to release at about 150 psi.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.6 Safety Valve

3. 3. What does the emergency brake system use to stop the vehicle when a brake failure occurs?

   Correct answer: Parts shared from the service and parking brake systems

   The emergency brake system is not entirely separate hardware; it borrows parts from both the service and parking brake systems to bring the vehicle to a stop if something fails.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - overview

4. 4. On an older vehicle that has one, where should the front brake limiting valve control be kept?

   Correct answer: In the 'normal' position for full stopping power

   Although the 'slippery' setting cuts front brake pressure in half, front-wheel braking is actually good under all conditions, so the control should stay in the 'normal' position for full stopping power.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.13 Front Brake Limiting Valve

5. 5. Why is it a bad habit to press and release the brake pedal unnecessarily?

   Correct answer: It can drain air faster than the compressor can rebuild it

   Each release lets compressed air escape, and repeatedly working the pedal can bleed air out faster than the compressor can replace it, until the pressure gets too low for the brakes to work.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.7 The Brake Pedal

6. 6. What is another name for the brake pedal in an air brake system?

   Correct answer: The foot valve or treadle valve

   The brake pedal is also referred to as the foot valve or treadle valve; pressing it harder sends more air pressure to the brakes.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.7 The Brake Pedal

7. 7. If the safety relief valve lets air escape, what does that indicate?

   Correct answer: Something is wrong and a mechanic should fix it

   The safety valve releasing air is a sign that something is wrong in the system, and the fault should be checked and repaired by a mechanic.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.6 Safety Valve

8. 8. How does the air brake system warn drivers behind you that you are braking?

   Correct answer: An air-operated switch turns on the brake lights

   An electric stop light switch that operates on air pressure turns on the brake lights whenever you apply the air brakes.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.12 Stop Light Switch

9. 9. How does an air-operated disc brake differ from an s-cam brake?

   Correct answer: It uses a power screw instead of an s-cam

   A disc brake uses a power screw in place of the s-cam; the slack adjuster turns the power screw, which clamps the rotor between the lining pads much like a large c-clamp.

   Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.8 Foundation Brakes

10. 10. In a dual air system, how quickly should pressure build from 85 to 100 psi with the engine at operating rpm?

    Correct answer: Within 45 seconds

    With the engine running at operating rpm, a dual air system should build pressure from 85 to 100 psi within 45 seconds; slower buildup could let pressure fall too low while driving.

    Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.3 Final Air Brake Check

11. 11. When checking a slack adjuster on an s-cam brake by hand, how much movement suggests it probably needs adjustment?

    Correct answer: More than about one inch of movement

    If pulling hard on the slack adjuster lets it move more than about one inch where the push rod attaches, it likely needs adjustment, and vehicles with too much slack can be very hard to stop.

    Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.2 Walk-around Inspection

12. 12. How do antilock brakes (ABS) help you during hard braking?

    Correct answer: They help prevent wheel lockup so you can keep steering

    ABS senses when a wheel is about to lock, reduces braking pressure to a safe level, and helps you avoid lockup; you may not stop any shorter, but you should be able to steer around an obstacle and avoid skids from overbraking.

    Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.2 Braking with Antilock Brakes