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

   Correct answer: Around 125 psi

   The governor halts the compressor at the cut-out point, which sits at about 125 psi.

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

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

   Correct answer: About 100 psi

   When pressure drops to the cut-in point of roughly 100 psi, the governor once again lets the compressor build air.

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

3. 3. Why do the brakes still work for several applications after the compressor stops running?

   Correct answer: The storage tanks keep a reserve of compressed air on hand

   The air tanks store enough compressed air to work the brakes several times, providing a cushion if the compressor stops.

   Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.3 Air Storage Tanks

4. 4. In an s-cam brake, what does the s-cam do when the brakes are applied?

   Correct answer: It forces the brake shoes outward against the inside of the drum

   Air moves the slack adjuster and twists the camshaft, rotating the s-cam so it pushes the shoes apart against the drum's inner surface.

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

5. 5. Why is it a mistake to pump the brake pedal on and off without need?

   Correct answer: It can release air faster than the compressor replaces it, dropping pressure too low to brake

   Each release lets some air out; pumping needlessly bleeds air faster than the compressor can refill, and if pressure falls too far the brakes stop working.

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

6. 6. How do air-operated disc brakes clamp the rotor rather than using an s-cam?

   Correct answer: A power screw turned by the slack adjuster squeezes the disc between caliper pads

   In disc brakes a power screw replaces the s-cam; the slack adjuster turns it, and it clamps the rotor between the pads much like a large c-clamp.

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

7. 7. What best describes a dual air brake system?

   Correct answer: Two separate air brake systems operated by a single set of controls

   A dual system has two independent air brake systems, each with its own tanks and lines, run from one set of brake controls; these are the primary and secondary systems.

   Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake

8. 8. In a dual air system at operating rpm, pressure should climb from 85 to 100 psi within about how long?

   Correct answer: 45 seconds

   For dual air systems, the build-up from 85 to 100 psi should happen within roughly 45 seconds at operating rpm.

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

9. 9. With a dual air system, how much pressure should you build up in each system before you start driving?

   Correct answer: 100 psi

   Allow the compressor to raise pressure to a minimum of 100 psi in both the primary and secondary systems before you drive.

   Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake

10. 10. With the engine off and brakes released, what is the most air a single vehicle should lose in one minute?

    Correct answer: Less than 2 psi

    On a fully charged single vehicle with the parking brake released, the leakage rate should stay under 2 psi per minute (under 3 psi for a combination).

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

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

    Correct answer: The s-cam drum brake

    Foundation brakes sit at each wheel, and the s-cam drum brake is the most widely used design.

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

12. 12. After fully applying the foot brake for one minute on a single vehicle, air loss above what rate is too much?

    Correct answer: More than 3 psi

    With the brakes applied, a single vehicle losing more than 3 psi per minute (or a combination more than 4 psi) has excessive leakage that must be fixed.

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