Idaho CDL — Air Brakes practice
Idaho CDL — Air Brakes: Safe Driving Practices practice test
Following distance, scanning, blind spots, using your lights, and the habits that prevent crashes and keep you in control.
Content reviewed against official driver handbooks · August 12, 2026
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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.
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1. Before you drive a vehicle with a dual air system, how much pressure does the manual want the compressor to build in both the primary and secondary systems?
Correct answer: At least 100 psi
The manual says to give the compressor time to bring both systems up to a minimum of 100 psi before driving. The low air warning shutting off is not enough by itself, since that only has to happen somewhere above 55 psi.
Source: Idaho Commercial Driver's License Manual — Section 5.2, Dual Air Brake
2. Idaho's inspection test section has you look for grease, oil and similar contaminants on brake linings, pads, drums and discs. Besides cutting braking friction, what other danger does it name?
Correct answer: A fire hazard
The section says contaminants on the brakes can cut braking friction, raising the chance of a crash, and that they can create a fire hazard as well. Where the brakes are accessible, you inspect the lining or pads and the drum or disc for them.
Source: Idaho Commercial Driver's License Manual — Section 11.5.2, Axle Components/Steering Axle
3. The final air brake check gives an engine speed for each of its two pressure buildup standards. Which pairing matches the manual?
Correct answer: Dual systems at operating rpm; pre-1975 single systems at an idle of 600 to 900 rpm
The dual-system check, 85 psi to 100 psi inside 45 seconds, is done with the engine at operating rpm. For pre-1975 single systems the typical requirement is a rise from 50 psi to 90 psi in no more than 3 minutes at an idle of 600-900 rpm.
Source: Idaho Commercial Driver's License Manual — Section 5.3.3, Step 7 Final Air Brake Check
4. How does the manual say an air compressor gets its drive from the engine?
Correct answer: Through gears or a v-belt
The compressor is linked to the engine by gears or by a v-belt. That is why a belt-driven compressor adds a belt check to the engine compartment step of your inspection.
Source: Idaho Commercial Driver's License Manual — Section 5.1.1, Air Compressor
5. Air disc brakes and s-cam brakes apply the brakes differently. What does the manual say the two designs share?
Correct answer: In both, air pressure works on a brake chamber and a slack adjuster
On air disc brakes, air pressure still works through a brake chamber and a slack adjuster, the same as on s-cam brakes. The difference comes afterward: a power screw does the job that the s-cam does in a drum brake.
Source: Idaho Commercial Driver's License Manual — Section 5.1.8, Foundation Brakes
6. During the emergency brake test on Idaho's vehicle inspection test, you fan the pressure down and the parking brake valve does not pop out until the gauge shows 15 psi. How does that compare with the manual's standard?
Correct answer: It is too low; the valve should close no lower than 20 psi and no higher than 45 psi
The manual sets the window at 20 to 45 psi, with 20 as the floor and 45 as the ceiling. A valve that holds until 15 psi falls outside that window.
Source: Idaho Commercial Driver's License Manual — Section 11.1.2, Air Brake or Hydraulic Brake Check
7. Which of these is NOT a cold-weather precaution found in the manual's Air Brakes section?
Correct answer: Pumping the brake pedal every few minutes to keep the air lines warm
The section never tells you to pump the pedal to warm the lines; needless pedal use only wastes air. Its cold-weather steps are the daily evaporator check, draining water from the tanks, and leaving the parking brakes off when very wet brakes could freeze.
Source: Idaho Commercial Driver's License Manual — Section 5.1.5, Alcohol Evaporator
8. The brakes-applied air leakage test has you release the parking brake with the engine off. What does the manual tell you to do, if needed, so the vehicle stays put?
Correct answer: Chock the wheels
With pressure at governor cutoff, you stop the engine, put chocks at the wheels if they are needed, let off the parking brake and then press the foot brake fully for one minute. The chocks hold the vehicle while its parking brakes are released.
Source: Idaho Commercial Driver's License Manual — Section 5.3.3, Step 7 Final Air Brake Check
9. A new driver lists four things he believes about air tank drain valves. Which one would the manual correct?
Correct answer: The valve is mounted at the top of the tank, above the water
Each tank's drain valve is in the bottom, because that is where water and oil settle. The other three statements agree with the manual.
Source: Idaho Commercial Driver's License Manual — Section 5.1.4, Air Tank Drains
10. The manual dates several air brake features. Which of these does it tie to vehicles built since 1994, not to vehicles made before 1975?
Correct answer: Automatic slack adjusters
Automatic slack adjusters have been fitted to every vehicle built since 1994, according to the manual. The cab-controlled front brake limiting valve and single air systems, with their 50-to-90 psi buildup standard, are both described as pre-1975 equipment.
Source: Idaho Commercial Driver's License Manual — Section 5.3.2, During Step 5 Walk-around Inspection
11. When air enters the brake chamber of an s-cam brake, the push rod is driven outward. Which part does the push rod move next?
Correct answer: The slack adjuster
Air entering the brake chamber drives the push rod outward, which moves the slack adjuster and twists the brake camshaft to turn the s-cam. The governor, safety valve and evaporator belong to the air supply side of the system.
Source: Idaho Commercial Driver's License Manual — Section 5.1.8, Foundation Brakes
12. Which pairing of a pressure with what the manual says happens there is correct?
Correct answer: About 150 psi — the safety relief valve usually opens
The safety valve normally opens at 150 psi. Around 100 psi is the cut-in point where pumping resumes, around 125 psi is cut-out, 55 psi is the low air warning mark, and tractor spring brakes come fully on somewhere from 20 to 45 psi.
Source: Idaho Commercial Driver's License Manual — Section 5.1.6, Safety Valve
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Every Idaho question is written from the official Idaho driver handbook and checked against its current edition. DMV Test Free is a free, independent study resource — not affiliated with any DMV or government agency. About DMV Test Free