CDL knowledge test

Air Brakes practice test

Required to clear the L restriction. Covers system parts, the seven-step air brake check, low-pressure warnings, spring brakes, and stopping distance.

67 questions available. A study sitting draws 25, and the App practice target: 80%; this is not an official test score. Your state sets the real test length.

Start a 10-question Air Brakes diagnostic Start the full Air Brakes simulator

Air Brakes questions with answers

Every question below carries the answer, the reason, and the section of the FMCSA CDL Manual or 49 CFR it comes from. Nothing here is a recalled exam question.

A dual air-brake vehicle has 65 psi in both systems. The parking brakes release. Is it ready to drive?
- ANo; build at least 100 psi in both systems and complete the brake checksCorrect
- BYes; parking-brake release proves readiness
- CYes; 60 psi is the normal driving pressure
- DYes; pressure in only one system matters
Parking-brake release does not prove safe operating pressure. Dual systems need at least 100 psi in both circuits before driving, plus the required checks.
Oregon DMV Commercial Driver Manual · 5.2 — Dual Air Brake Systems · verified 2026-09-07
What does a low-air-pressure warning tell a driver?
- AThe parking brakes passed inspection
- BStored air has fallen to the warning levelCorrect
- CThe compressor has reached cutout
- DThe brake linings are new
The warning signals falling stored air. Stop safely when it appears while driving. Check the vehicle specifications and applicable manual for the test threshold; do not assume one setting fits every vehicle.
Oregon DMV Commercial Driver Manual · 5.3.3 Final Air Brake Check; 5.1.11 Low Air Pressure Warning · verified 2026-09-07
In an air-brake system, what does the air-compressor governor control?
- AThe maximum speed of the vehicle
- BWhen the air compressor pumps air into tanksCorrect
- CThe engine RPM
- DThe fuel injection system
The governor controls when the compressor pumps air into storage tanks. The current Oregon CDL manual gives about 125 psi for cut-out and about 100 psi for cut-in; during a vehicle check, use the manufacturer’s specified pressures.
Oregon Commercial Driver Manual · 5.1.2 Air Compressor Governor; 5.3.3 Final Air Brake Check · verified 2026-09-07
The safety valve releases excess air at approximately:
- A100 PSI
- B125 PSI
- C150 PSICorrect
- D175 PSI
The safety valve protects the tank from over-pressurization by releasing air at about 150 PSI.
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.1.6 — Safety Valve; Oregon Manual § 5.1.6 cross-check · verified 2026-09-19
When testing air leakage rate on a single vehicle, the maximum allowable loss with engine off and service brakes applied is:
- A1 PSI per minute
- B2 PSI per minute
- C4 PSI per minute
- D3 PSI per minuteCorrect
With a fully charged system and service brakes applied, air loss should be no more than 3 PSI per minute (single vehicle) or 4 PSI (combination).
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.3.3 — Applied Air-Leak Test; Oregon Manual § 5.3.3 cross-check · verified 2026-09-19
Tractor and straight-truck spring brakes typically apply fully as pressure drops into which range?
- A10-15 PSI
- B20-45 PSICorrect
- C50-60 PSI
- D80-90 PSI
The typical range is 20–45 psi; check the vehicle specification. Application does not guarantee effective stopping if the brakes are out of adjustment. Stop safely at the low-air warning instead of waiting for spring brakes.
Oregon DMV Commercial Driver Manual · 5.1.14 Spring Brakes · verified 2026-09-07
Brake lag in an air brake system is:
- AGovernor delay time
- BWear in the brake pads
- CThe time for air to reach the brakesCorrect
- DCompressor warm-up period
Brake lag is the delay after the brake pedal is pushed while air flows through the lines and applies the brakes. The current Oregon CDL manual says this takes one-half second or more, so it adds to stopping distance.
Oregon Commercial Driver Manual · 5.4.4 Stopping Distance — Brake Lag · verified 2026-09-07
With the vehicle secured against movement, how do you test the low-air warning?
- AKeep the engine running at a fast idle and let pressure build until the governor cuts out, then note the pressure on the gauge
- BOpen the tank drain valves fully and wait for the parking valve to pop out
- CDisconnect the trailer supply line and listen for the warning buzzer
- DEngine off and power on, step on and off the brake pedal to lower pressure, and watch the warningCorrect
Pedal applications lower stored air so the warning can be checked. Use the applicable manual and vehicle specifications for its threshold. Parking-valve pop-out is a separate check.
Oregon DMV Commercial Driver Manual · 5.3.3 — Test Low Pressure Warning Signal · verified 2026-09-29
During the applied leakage test, air loss for a combination vehicle should not exceed:
- A2 PSI per minute
- B3 PSI per minute
- C5 PSI per minute
- D4 PSI per minuteCorrect
Combination vehicles can leak up to 4 PSI per minute with service brakes applied; single vehicles up to 3 PSI per minute.
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.3.3 — Applied Air-Leak Test; Oregon Manual § 5.3.3 cross-check · verified 2026-09-19
For an emergency stop without ABS, what sequence describes stab braking?
- ALightly tapping the brakes continuously
- BApply fully, release at wheel lock, and reapply after the wheels rollCorrect
- COnly using the parking brake to slow the vehicle
- DPumping the brakes rhythmically
This non-ABS method requires wheel rotation to resume before the next full application. It is not light rhythmic pumping or parking-brake use.
Oregon DMV Commercial Driver Manual · 2.17.2 · verified 2026-09-07
What is the purpose of the air pressure governor on an air brake system?
- ATo prevent brake fade when the vehicle is descending long steep downgrades
- BTo regulate the air pressure delivered to each individual brake chamber
- CTo stop compressor pumping at cut-out and allow it to restart at cut-inCorrect
- DTo apply the emergency brakes automatically whenever the air pressure drops too low
The governor controls when the air compressor pumps air into the reservoirs: it stops pumping at cut-out and allows pumping again at cut-in. Use the manufacturer's specified pressures when inspecting the vehicle; Oregon's manual gives about 125 psi cut-out and about 100 psi cut-in as typical values.
Oregon Commercial Driver Manual · 5.1.2 Air Compressor Governor; 5.3.3 Final Air Brake Check · verified 2026-09-07
In the static air-leak test, which stated limit is correct for a combination vehicle?
- ALess than 3 psi in one minuteCorrect
- BLess than 1 psi in one minute
- CLess than 5 psi in one minute
- DLess than 10 psi in one minute
For the static test, the manual states less than 3 psi in one minute for a combination vehicle and less than 2 psi for a single vehicle. The separate applied test uses limits of 4 psi for a combination vehicle and 3 psi for a single vehicle.
FMCSA-hosted Commercial Driver License Manual (July 2014) · 5.3.3 — Test Air Leakage Rate · verified 2026-09-07
The 'applied leakage test' for a combination vehicle should not show more than what pressure loss per minute?
- A6 psi
- B3 psi
- C4 psiCorrect
- D10 psi
With the service brakes fully applied, a combination vehicle should lose no more than 4 psi in one minute in the manual’s applied-leakage check. The single-vehicle limit is 3 psi in one minute. A greater drop exceeds the check limit and needs inspection.
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.3.3 — Applied Air-Leak Test; Oregon Manual § 5.3.3 cross-check · verified 2026-09-19
What is 'brake lag' and why is it important?
- AThe lag between steering input and vehicle response
- BThe delay between pressing the pedal and the brakes applyingCorrect
- CBrake fade on downgrades, a separate heat-related issue
- DThe time it takes for brakes to heat up
Brake lag is the delay between pressing the brake pedal and the brakes working. With air brakes, it takes one-half second or more for air to flow through the lines to the brakes. That delay adds brake-lag distance to perception, reaction, and braking distance, so leave enough following space.
Oregon Commercial Driver Manual · 5.4.4 Stopping Distance — Brake Lag · verified 2026-09-07
On a newer tractor, which control normally applies the tractor parking brakes?
- APulling the diamond-shaped yellow knobCorrect
- BPressing the brake pedal firmly
- CEngaging the service brake with the foot
- DPulling the red 8-sided knob
On newer vehicles, pulling the yellow diamond-shaped control applies the tractor parking brakes. The red eight-sided control manages trailer air supply. Older vehicles may use a different-shaped parking control, so identify the labeled control for that vehicle.
FMCSA CDL Manual · 5.1.15 — Parking Brake Controls · verified 2026-09-07
Before driving a vehicle with a dual air-brake system, where must pressure reach at least 100 psi?
- AOnly the primary circuit
- BOnly the secondary circuit
- CBoth primary and secondary circuitsCorrect
- DNeither circuit if the warning is off
Check both gauges. An extinguished warning alone does not establish adequate driving pressure.
Oregon DMV Commercial Driver Manual · 5.2 — Dual Air Brake Systems · verified 2026-09-07
Spring brakes are designed to:
- ASupplement the service brakes on grades
- BApply automatically when air pressure drops too lowCorrect
- CPrevent wheel lockup
- DApply additional force during hard stops
Air pressure holds spring brakes released. Loss of air lets the springs apply. Their braking force depends on proper brake condition and adjustment; automatic application is not a guarantee of a safe stop.
Oregon DMV Commercial Driver Manual · 5.1.14 Spring Brakes · verified 2026-09-07
For a dual air system with standard-size reservoirs, what is the usual 85-to-100-psi buildup check at operating engine rpm?
- A10 seconds
- B10 minutes
- C3 minutes
- D45 secondsCorrect
The usual check is within 45 seconds at operating rpm. Larger reservoirs may have a different permissible buildup time; use the manufacturer specifications. A failed applicable check needs repair before driving.
Oregon DMV Commercial Driver Manual · 5.3.3 Final Air Brake Check; 5.1.11 Low Air Pressure Warning · verified 2026-09-07
The air compressor governor stops the compressor from pumping air (cut-out) at about:
- A125 psiCorrect
- B75 psi
- C50 psi
- D200 psi
The governor cuts out around 125 psi to stop pumping and cuts back in near 100 psi, keeping tank pressure in a safe working range.
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.1.2 — Air Compressor Governor; Oregon Manual § 5.1.2 cross-check · verified 2026-09-19
On a tractor, the spring (parking/emergency) brakes will come on automatically when air pressure drops to about:
- A100 to 110 psi
- B75 to 85 psi
- C20 to 45 psiCorrect
- D5 to 10 psi
A typical application range is 20–45 psi, subject to the vehicle specification. Brake condition and adjustment still matter. Stop safely at the low-air warning; do not wait for this automatic application.
Oregon DMV Commercial Driver Manual · 5.1.14 Spring Brakes · verified 2026-09-07

What this test is drawn from

2.16.2–2.16.4
2.16.4
2.17.2
2.18.5/2.18.7 ABS malfunction; 5.1.16 Antilock Braking Systems
2.6.1 — Stopping Distance
49 CFR 383.95(a); 49 CFR 383.153(a)(10)(i)
5.1.14 Spring Brakes
5.1.14 and 5.3.3 — Spring Brakes
5.1.14 — Spring Brakes
5.1.15 — Parking Brake Controls
5.1.2 Air Compressor Governor; 5.3.3 Final Air Brake Check
5.1.7 The Brake Pedal; 5.4.7 Low Air Pressure
5.1.9 and 5.3.3 — Air Compressor Governor
5.1.x Test Air Leakage Rate (Section 5.3.3 test uses different conditions)
5.2 and 5.3.3 — Dual Systems and Final Air Brake Check
5.2 — Dual Air Brake Systems
5.3.3 Final Air Brake Check; 5.1.11 Low Air Pressure Warning
5.3.3 — Test Air Leakage Rate
5.3.3 — Test Low Pressure Warning Signal
5.4.4 Stopping Distance — Brake Lag
5.4.5 — Brake Fading or Failure
5.4.8 — Parking Brakes
Section 11: Vehicle Inspection Test — Brake Lines / Hoses / Leaks; Air/Electric Lines
Section 2.18.3: How to Know If Your Vehicle Is Equipped with ABS
Section 2.6.1: Stopping Distance
Section 2.6.6: Mountain Driving; Section 5.4.5: Brake Fading or Failure
Section 5, Figure 5.4: Air brake system components and location; historical July 2014 edition
Section 5.1 — Air Brake System Components; Section 5.2 — Dual Air Brake
Section 5.1.10: Application Pressure Gauge
Section 5.1.15: Modulating Control Valves
Section 5.1.2 — Air Compressor Governor; Oregon Manual § 5.1.2 cross-check
Section 5.1.2: Air Compressor Governor; Section 5.3.3: Final Air Brake Check
Section 5.1.2: Air Compressor Governor; Section 5.3.3: Step 7 Final Air Brake Check
Section 5.1.3: Air Storage Tanks
Section 5.1.4: Air Tank Drains
Section 5.1.5: Alcohol Evaporator
Section 5.1.6 — Safety Valve; Oregon Manual § 5.1.6 cross-check
Section 5.1.6: Safety Valve
Section 5.2: Dual Air Brake
Section 5.3.1: During Step 2 Engine Compartment Checks
Section 5.3.2: During Step 5 Walk-around Inspection
Section 5.3.3 — Applied Air-Leak Test; Oregon Manual § 5.3.3 cross-check
Section 5.3.3: Step 7 Final Air Brake Check
Section 5.4.4 — Brake Lag; Oregon Manual § 5.4.4 cross-check
Section 6.2.1: Trailer Hand Valve
§393.51(c)(2): Air-brake warning signal for power units not subject to FMVSS No. 121
§396.7(a)–(b): Unsafe operations forbidden and nearest-repair exception

The other CDL knowledge tests

Lea este examen en español

Air Brakes practice test

Air Brakes questions with answers

A dual air-brake vehicle has 65 psi in both systems. The parking brakes release. Is it ready to drive?

What does a low-air-pressure warning tell a driver?

In an air-brake system, what does the air-compressor governor control?

The safety valve releases excess air at approximately:

When testing air leakage rate on a single vehicle, the maximum allowable loss with engine off and service brakes applied is:

Tractor and straight-truck spring brakes typically apply fully as pressure drops into which range?

Brake lag in an air brake system is:

With the vehicle secured against movement, how do you test the low-air warning?

During the applied leakage test, air loss for a combination vehicle should not exceed:

For an emergency stop without ABS, what sequence describes stab braking?

What is the purpose of the air pressure governor on an air brake system?

In the static air-leak test, which stated limit is correct for a combination vehicle?

The 'applied leakage test' for a combination vehicle should not show more than what pressure loss per minute?

What is 'brake lag' and why is it important?

On a newer tractor, which control normally applies the tractor parking brakes?

Before driving a vehicle with a dual air-brake system, where must pressure reach at least 100 psi?

Spring brakes are designed to:

For a dual air system with standard-size reservoirs, what is the usual 85-to-100-psi buildup check at operating engine rpm?

The air compressor governor stops the compressor from pumping air (cut-out) at about:

On a tractor, the spring (parking/emergency) brakes will come on automatically when air pressure drops to about:

What this test is drawn from

The other CDL knowledge tests