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newSchoolWorks.com › VoTechWorks › Transportation › Commercial Driving (CDL) › Pre-Trip and Post-Trip Vehicle Inspection (CDL) › Lesson 3
TRN-CDL23-02 · Lesson 3 · 5 clock hours · lab

Brake Systems, Tires, Wheels, and Lighting Walk-Around

Objective. Students will perform the exterior walk-around inspection steps covering air brake components, brake drums and linings, tires, wheels, lug nuts, and all required lights and reflectors, verbally explaining each check and correctly identifying introduced defects.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why This Walk-Around Matters

Federal regulations require every commercial driver to inspect the vehicle before driving and again after completing a trip. A missed defect is not just a test failure—it can mean a roadside out-of-service order, a crash, or a fatality. The walk-around covering brake systems, tires, wheels, and lighting is the portion inspectors and examiners watch most closely because defects in these areas are the leading causes of commercial vehicle crashes and citations.

Safety Before You Start

Before walking under or around a large vehicle, set the parking brake and chock the wheels if the vehicle is on a grade. Place safety cones around the work zone to alert others that an inspection is in progress. Never reach into wheel wells or near moving components with the engine running unless the specific check requires it. Wear appropriate footwear—no open-toed shoes. If you must go under the vehicle, confirm it is on firm, level ground and cannot roll.

Air Brake Components

On a Class A tractor-trailer equipped with air brakes, you must check several components as part of the exterior walk-around.

Air lines and hoses: Look for cracks, cuts, abrasions, or improper splices. Hoses must not be rubbing on frame components or hot surfaces.

Brake chambers: These are round metal canisters mounted near each axle. Look for cracks, dents, or loose mounting bolts. The pushrod (a metal rod extending from the chamber) should not appear to be over-extended; excessive pushrod travel indicates worn linings or an out-of-adjustment brake.

Slack adjusters: These mechanical arms connect the pushrod to the brake camshaft. On manual slack adjusters, look for the proper angle—roughly 90 degrees when brakes are released. Automatic slack adjusters should maintain adjustment without manual intervention, but you still inspect them visually for damage and loose pins.

Brake drums and linings: Look through the holes in the wheel or around the drum edge for visible lining material. Drums must not have cracks that extend to the open edge or deep scoring. Lining must not be dangerously thin, loose, or soaked with oil or grease.

In-Cab Air Brake Checks (Engine Running)

Although performed from the cab, these checks are part of the required pre-trip sequence and you will record the results:

Tires and Wheels

Tread depth: Front (steering) tires require more tread depth than rear drive or trailer tires. Use a tread depth gauge at the major grooves. Your CDL manual and the Federal Motor Carrier Safety Regulations specify minimum depths; know these numbers and always record your actual reading.

Tire condition: Walk each tire and look for cuts, cracks in the sidewall, bulges, blisters, or any exposed fabric or cord. Any of these conditions is an automatic out-of-service item—do not drive the vehicle.

Tire inflation: Tires must be properly inflated. Under-inflation and over-inflation both cause dangerous handling and premature wear. If a tire appears flat or noticeably lower than the others, flag it immediately.

Wheels and lug nuts: Look at each lug nut. Fresh rust streaks radiating outward from a lug nut indicate that it has been working loose and must be tightened before operating. Bent, cracked, or missing lug nuts are out-of-service conditions. Also look at the rim for cracks or welds that appear improper.

Lights and Reflectors

Every required light and reflector must be present, clean, and functional. Work systematically—front to driver's side to rear to passenger's side—so nothing is missed.

Required lights on a combination vehicle include: headlights (low and high beam), front turn signals, identification lights (three amber lights on the front of the cab if applicable), side marker lights, clearance lights, rear turn signals, brake lights, tail lights, reverse lights, and hazard flashers.

Reflectors: Required reflectors must be the correct color for their location (amber on the sides toward the front, red toward the rear) and must not be cracked or missing.

Inspection technique: One student operates the controls from the cab; the other walks the exterior and marks each light as working, non-working, or missing on the scoring rubric. Then partners switch so each person practices both roles. A single non-working or missing required light is a defect that must be reported; certain missing lights are out-of-service conditions under federal regulations.

Documenting Defects

Every defect you find must be recorded on the Driver Vehicle Inspection Report (DVIR). Mechanics must review and either repair the defect or certify that no repair is needed before the next driver signs off. If an out-of-service defect is found, the vehicle must not be moved until corrected. On your skills test, verbally stating each check and correctly identifying staged defects demonstrates that you understand—not just memorize—the process.

Key terms10

Slack adjusterA mechanical arm that connects the air brake chamber pushrod to the brake camshaft; it multiplies force and, on automatic versions, maintains proper brake adjustment.
Pushrod travelThe distance the rod extending from the brake chamber moves when the brake is applied; excessive travel indicates worn linings or an out-of-adjustment brake.
Low-pressure warningA required audible or visual alert that activates when air system pressure drops to a level where safe braking may be compromised, as specified in the CDL manual.
Static leakage testAn air brake test performed with the engine off, system fully charged, and brakes released—then applied—to verify that pressure loss per minute does not exceed the limit in the CDL manual.
Tread depth gaugeA small measuring tool inserted into a tire's major tread groove to give a precise depth reading in 32nds of an inch, used to determine whether a tire meets minimum legal depth.
Out-of-service conditionA defect serious enough that the vehicle must not be operated until it is repaired, as defined by federal or state regulations and the North American Standard Out-of-Service Criteria.
Brake drumThe rotating metal cylinder against which brake shoes press to slow the wheel; inspected for cracks, deep scoring, and contamination during the walk-around.
Driver Vehicle Inspection Report (DVIR)The written record a driver must complete after each trip noting any defects or deficiencies found or observed during operation; required by federal regulation.
Clearance lightsLights mounted at the outermost edges of the vehicle's width and height to show other drivers the vehicle's size; required on vehicles wider than a specified threshold.
Governor cut-outThe air pressure level at which the compressor stops pumping and the air dryer purges; the system should reach this pressure within the time specified in the CDL manual.

Worked examples4

1. You use a tread depth gauge on a front (steering) tire and get three readings from different grooves: 6/32", 5/32", and 4/32". The CDL manual states the minimum for steering tires is 4/32". Is this tire acceptable, and what do you record?
  1. Step 1: Identify the minimum tread depth required for a steering axle tire: 4/32 inch.
  2. Step 2: Compare each reading to the minimum. Readings are 6/32, 5/32, and 4/32—all are at or above 4/32.
  3. Step 3: The lowest reading (4/32) exactly meets the minimum—it is not below it.
  4. Step 4: Record all three readings on the data sheet and note the tire as marginally acceptable but monitor closely; flag it so the next driver is aware it is at the limit.
Answer: The tire meets the minimum standard at its lowest point (4/32") and is acceptable for operation, but the finding must be recorded on the data sheet because it is at the threshold. Any reading below 4/32" on a steering tire would be an out-of-service condition.
2. During the static leakage test on a combination vehicle, you record the following air pressure readings: 120 psi at the start of the one-minute test, 113 psi at the end (brakes released). Your CDL manual states the maximum allowable pressure drop for a combination vehicle is 4 psi per minute. Does the vehicle pass?
  1. Step 1: Calculate the pressure drop: 120 psi − 113 psi = 7 psi dropped in one minute.
  2. Step 2: Compare the drop to the allowable limit: 7 psi > 4 psi.
  3. Step 3: The vehicle exceeds the allowable leakage rate.
Answer: The vehicle fails the static leakage test. A 7 psi drop in one minute exceeds the 4 psi per minute limit for a combination vehicle. This is a defect that must be recorded and repaired before the vehicle is operated.
3. A rear drive tire on a tractor shows these tread depth readings across three grooves: 3/32", 4/32", and 5/32". The CDL manual minimum for non-steering tires is 2/32". Is this tire out of service, and what is the lowest acceptable reading for this axle position?
  1. Step 1: Identify the minimum for non-steering (drive/trailer) tires: 2/32 inch.
  2. Step 2: Compare all readings to 2/32: 3/32, 4/32, and 5/32 are all above 2/32.
  3. Step 3: No reading falls below the minimum, so the tire is not an out-of-service condition based on tread depth alone.
  4. Step 4: Record all readings; continue inspecting the tire for cuts, bulges, exposed cord, and sidewall damage before concluding the tire check.
Answer: The drive tire passes the minimum tread depth requirement (lowest reading 3/32" versus the 2/32" minimum). The lowest acceptable reading for this axle position is 2/32". Tread depth alone does not end the inspection—visible damage must still be checked.
4. You fan the service brake pedal during the low-pressure warning check. The buzzer activates when the gauge reads 60 psi. A classmate says it should have activated at a higher pressure. Who is correct, and why does it matter?
  1. Step 1: Understand the purpose of the low-pressure warning: it must alert the driver before pressure falls to a level where braking effectiveness is seriously compromised.
  2. Step 2: The CDL manual states the warning must activate before pressure drops to a specific threshold (consult your state CDL manual for the exact value, which is typically around 60 psi or higher depending on the system).
  3. Step 3: If the warning activates at or above the specified threshold, the system is working correctly. If it activates below that threshold—or not at all—the vehicle has a defect.
  4. Step 4: Verify what your specific CDL manual and vehicle specs require. Record the actual activation pressure on the data sheet and compare to the acceptable range.
Answer: Whether 60 psi is correct depends on the vehicle and what your CDL manual specifies as the minimum activation pressure. The key point is that the warning must activate before pressure falls to a dangerously low level. If 60 psi is at or above the required threshold, the system passes. If the warning activates below the threshold—or fails to activate—it is a defect requiring repair before operation.
In class: Air brake focus lab: with the engine running, students check air pressure build-up rate, test the low-pressure warning buzzer/light by fanning the brake pedal, and perform the static leakage test, recording results on a data sheet and comparing to acceptable ranges from the CDL manual. · Tire and wheel station rotation: students use a tread depth gauge to measure tread depth on front and rear tires, check for visible cracks, bulges, or exposed cords, and check that all lug nuts appear secure and have no rust streaks; each student records findings and flags any out-of-service condition. · Lights and reflectors check: working in pairs, one student activates each light function (headlights low/high, turn signals, brake lights, marker lights, clearance lights, hazard flashers) from the cab while the partner walks the exterior with the CDL scoring rubric and marks each light as working, non-working, or missing; partners switch roles.
Materials: Class A tractor-trailer (training vehicle), tread depth gauges, air pressure gauges, laminated brake and tire checklist, CDL skills test scoring rubric, data recording sheets, safety cones to mark work zone around vehicle
Check: Students must correctly identify all three instructor-staged defects (one brake-related, one tire/wheel-related, one lighting-related) on a scored practice run and score at least 80% on the walk-around section of the CDL rubric; any missed out-of-service condition requires immediate repeat of that station.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Pre-Trip Inspection
1. During the exterior walk-around, you notice fresh rust streaks radiating outward from two lug nuts on a drive axle wheel. What does this most likely indicate?
Air Brakes
2. You are performing the static air leakage test on a combination vehicle with brakes released. After one minute, how much pressure loss is generally considered the maximum allowable according to CDL guidance?
Pre-Trip Inspection
3. Which of the following tire conditions is an automatic out-of-service defect that means the vehicle must not be driven until repaired?
Pre-Trip Inspection
4. When performing the lights check in pairs, why should partners switch roles so each student works both the cab controls and the exterior walk?
Air Brakes
5. What is the primary purpose of checking pushrod travel on air brake chambers during the walk-around inspection?
Pre-Trip Inspection
6. A student finds a brake drum with a crack that extends to the open edge of the drum. What is the correct action?
Pre-Trip Inspection
7. Which of the following best describes the function of clearance lights on a large commercial vehicle?
Pre-Trip Inspection
8. During the skills test walk-around, the examiner has staged a defect: one brake chamber has a visible crack in its housing. A student walks past it without comment. What is the most likely result?
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