Lesson 1.1 — Brakes

Why this matters

Brake problems are the #1 reason trucks get put out of service on the road. CVSA's 2024 Brake Safety Week found brake violations on about 1 in every 8 trucks they inspected.

If your brakes fail inspection, you stop. The load stops. You don't drive until it's fixed. This module shows you the four checks to do before every trip so it doesn't happen to you.

Watch this first (15 minutes)

Captions in English are available. Click CC on the player.

PLACEHOLDER — final video pending topic-matched curation

The LABS check — four steps before every trip

LABS is what most CDL instructors teach. It is the four checks every commercial driver does on the air brake system before driving. Do them in order.

L Leaks

  1. Park on level ground. Chock the wheels.
  2. Engine off. Key on.
  3. Release the parking brake.
  4. Hold the foot brake down hard for one minute.
  5. Watch the air gauge. Listen.

Allowed loss: 3 PSI in one minute on a tractor alone, 4 PSI on a tractor with trailer.

If air drops more than that — STOP. Write it on the DVIR. Tell dispatch.

A Alarms

  1. Pump the foot brake to drop the air pressure.
  2. Watch and listen.

What must happen: the low-air warning light and buzzer must come on before pressure drops below 60 PSI.

If they do not come on — STOP. Write it on the DVIR.

B Buttons

  1. Keep pumping the foot brake.
  2. Watch the dash buttons (yellow tractor button, red trailer button).

What must happen: both buttons must pop out before pressure drops below 20 PSI.

If a button does not pop — STOP.

S Service brake

  1. Build pressure back to 100+ PSI.
  2. Release the parking brake.
  3. Roll forward at about 5 mph.
  4. Press the foot brake hard.

What must happen: the truck stops straight, smooth, no pulling to one side.

If it pulls or shakes — STOP.

Red flags during your walk-around

Before you climb in the cab, walk the truck. Look at every brake.

Cracked brake drum — if you see a crack on the outside of the drum that goes more than halfway across, the drum is out of service. STOP.

Slack adjuster more than 2 inches — push the slack adjuster by hand. On a Type 30 chamber, if it moves more than 2 inches, the brake is out of adjustment. STOP.

Wet or oily brake chamber — oil or air leaking from the chamber means the seal is gone. STOP.

ABS warning lamp stays on — the ABS light should come on when you turn the key, then go out within a few seconds. If it stays on, the ABS is broken. STOP.

What gets you written up

These are the FMCSA brake violation codes that show up most often on roadside inspection reports. Each one means a specific thing. Knowing what the inspector is looking at helps you fix it first.

Code What it means What the inspector wrote
393.47(e)Slack adjuster pulls more than 2 inches"Brake out of adjustment"
393.53(b)Automatic slack adjuster failed to adjust"Auto-adjuster inoperative"
393.48(a)A brake on the truck is not working at all"Inoperative brake"
393.45(a)Air hose or line cut, worn, or leaking"Brake hose / tubing chafing"
396.3(a)(1)20% or more of the brakes are defective (out of service)"Defective brakes — OOS"

What protects you

  1. Walk around the truck every trip. Every single trip. Look at every wheel.
  2. Listen for hissing. A hiss you hear at the wheel means an air leak.
  3. Watch the air gauge. If it drops fast while you are sitting still, something is wrong.
  4. Write it on the DVIR. If you see something — write it down. The DVIR protects you in court if something goes wrong later.

Next step

Take the short quiz below. You need 4 of 5 correct (80%) to complete this module. You can retake it as many times as you need.

📋 Sample Quiz Questions (Preview)

These are the questions on the quiz at the end of this lesson. The actual quiz is taken after logging in. Correct answer marked with ✓.

Question 1: Q1: LABS L

What does the L in the LABS air-brake check stand for?

  • ○ Lights
  • ✓ Leaks
  • ○ Levers
  • ○ Load
Why: Review the LABS check section. <strong>L</strong> stands for Leaks.
Question 2: Q2: Allowed air loss

You hold the foot brake down for one minute with the engine off and the parking brake released. The maximum allowed air loss on a tractor with a trailer is:

  • ○ 1 PSI
  • ✓ 4 PSI
  • ○ 10 PSI
  • ○ 20 PSI
Why: On a combination (tractor with trailer), air loss must not exceed 4 PSI in one minute. On a tractor alone the limit is 3 PSI.
Question 3: Q3: Low air warning

The low air warning light and buzzer must come on before air pressure drops below:

  • ○ 100 PSI
  • ○ 80 PSI
  • ✓ 60 PSI
  • ○ 20 PSI
Why: The warning must trigger before pressure drops below 60 PSI.
Question 4: Q4: Buttons pop

The yellow tractor button and red trailer button on the dash must pop out before air pressure drops below:

  • ○ 60 PSI
  • ○ 45 PSI
  • ✓ 20 PSI
  • ○ 0 PSI
Why: The buttons must pop before air drops below 20 PSI. They can pop anywhere between 20 and 45 PSI, but 20 is the floor.
Question 5: Q5: First action on a defect

You find a brake problem during your pre-trip inspection. What is the first thing you do?

  • ○ Drive carefully to the next yard
  • ✓ Write it on the DVIR and tell dispatch
  • ○ Tell the next driver
  • ○ Ignore it if the load is hot
Why: Always write it on the DVIR and tell dispatch first. The DVIR is what protects you legally.

End of preview. The actual quiz requires login to record a grade.

Last modified: Tuesday, 19 May 2026, 8:30 PM