Idaho Air Brakes Practice Test

Required if the vehicle you'll drive has air brakes — which in practice means nearly every combination vehicle and most straight trucks. The test covers the parts of an air-brake system (compressor, governor, tanks, valves), the specific pressures where things happen (cut-in, cut-out, low-air warning, spring-brake pop-out), and the inspection sequence for leaks and warning devices. Fail this knowledge test or take the skills test in a vehicle without air brakes, and your license carries an air-brake restriction that keeps you out of most trucking jobs.

Who needs it: Anyone who'll drive a vehicle with air brakes. Skipping it puts an L or Z restriction on the license.

Sample practice questions

The brake pedal is also called the:

Answer: Foot valve or treadle valve — The brake pedal is the foot valve or treadle valve.

When you press the brake pedal in an S-cam brake, the correct order of action is:

Answer: Brake chamber → push rod → slack adjuster → S-cam → shoes → drum — Air enters the chamber, pushes the rod, moves the slack adjuster, turns the S-cam, which spreads the shoes against the drum.

At 55 mph, brake lag adds about how much distance?

Answer: 32 feet — Brake lag adds about 32 feet at 55 mph.

Common mistakes on the Air Brakes test

  • Mixing up the pressure numbers — cut-out (~125 psi), cut-in (~100 psi), low-air warning (~60 psi) are all tested
  • Confusing spring brakes (parking/emergency) with service brakes — which system does what, and when
  • Forgetting the air-leak rate limits for the applied and released tests during inspection
  • Assuming ABS changes how you brake — the test wants normal braking with ABS, not pumping

Studying with the full Truckcess course does double duty: the theory counts toward the federal ELDT requirement (Truckcess is listed on the FMCSA Training Provider Registry).

Official source: Idaho Transportation Department

CDL licensing in Idaho is administered by the Idaho Transportation Department. Fees, scheduling, forms, and the official Idaho CDL manual live on the official site: Idaho ITD — official CDL page (verified 2026-08-09).

Truckcess is not affiliated with or endorsed by the Idaho Transportation Department. Our practice tests cover the federal CDL knowledge standards (49 CFR Part 383) that every state exam is built on; exact question counts, fees, and procedures vary by state and change over time — the official site is always the authority.

More Idaho practice tests

Full prep courses are one-time from $59 with 12 months of access — mytruckcess.com/cdl.

CDL practice tests by state · Meet the Course Director · The completion certificate