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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Preventive Maintenance for Diesel & Heavy Trucks › Lesson 9
TRN-DIES20-06 · Lesson 9 · 20 clock hours · lab

Brake System PM Inspection

Objective. Students will perform an air brake PM inspection including measuring lining thickness, checking slack adjuster free stroke against Federal Motor Carrier Safety Administration (FMCSA) out-of-service criteria, inspecting air chambers and hardware, and recording all findings on a standard brake inspection report.

⚠ 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.

Brake System PM Inspection — Air Brakes on Class 8 Trucks

Air brakes are the primary stopping system on heavy trucks, buses, and combinations. A failed or worn air brake component can cause a loss-of-control crash. Federal law gives inspectors the authority to place a vehicle out of service immediately when brake conditions meet defined thresholds. As a diesel technician, you must know those thresholds cold, measure accurately, and document honestly.

Safety First: LOTO and Wheel Chocks

Before any brake work begins, the vehicle must be on approved jack stands rated for its weight — never on the jack alone. Chock all wheels that remain on the ground. Apply the full lockout/tagout (LOTO) procedure: deplete all air from the system by draining the reservoirs and cycling the brakes until pressure reads zero on the dash gauge. Cage or remove the spring brake actuators before working near rear chambers — a spring brake stores enough energy to crush a hand. Wear safety glasses and gloves throughout. Do not crawl under a vehicle supported only by a floor jack.

The FMCSA Out-of-Service Criteria

The Federal Motor Carrier Safety Administration (FMCSA) publishes out-of-service (OOS) criteria that roadside inspectors use during Commercial Vehicle Safety Alliance (CVSA) inspections. Technicians performing PM inspections use the same benchmarks. A vehicle that meets an OOS condition must not be returned to service until the defect is corrected. The two most common brake OOS triggers you will check today are lining thickness and pushrod travel (slack adjuster free stroke).

Lining Thickness Measurement

Brake lining wears from friction. When it becomes too thin, it cannot absorb and dissipate the heat needed to stop the vehicle safely, and the metal shoe can score the drum. Use a brake-lining gauge (or a ruler at the thinnest point) to measure the remaining lining. Always measure at the thinnest location on the shoe — usually the heel or toe.

FMCSA OOS threshold (air brakes, drum type): Lining worn to less than ¼ inch (6.4 mm) on a steering axle, or less than ¼ inch on any other axle with a combination of ¼ inch or less on two or more adjacent shoes on the same axle, places the vehicle out of service. Always confirm the current OOS criteria card because values are periodically reviewed. OEM minimum thickness may be more conservative than the federal floor — use whichever limit is more restrictive.

Record each measurement on the brake inspection report: axle number, wheel position (left/right), and lining thickness at the thinnest point. A complete report covers every shoe on every axle.

Slack Adjuster and Pushrod Travel

The slack adjuster converts the linear push of the air chamber pushrod into rotational force on the S-cam. As lining wears, the cam must rotate farther to apply the brakes, which means the pushrod travels farther before generating clamping force. Excessive pushrod travel indicates worn lining, an out-of-adjustment slack adjuster, or a failing automatic slack adjuster (ASA).

Measuring free stroke: With the air system charged and brakes released, mark the pushrod at the air chamber face. Apply approximately 90 psi to the brake system (or use a helper to make a full brake application), then measure how far the pushrod moved. This is applied stroke. The FMCSA OOS limits for pushrod travel depend on the size of the air chamber (Type 20, Type 24, Type 30, etc.) — the OOS criteria card lists the limit for each type. A common limit for a Type 30 chamber is 2 inches (50.8 mm), but always verify against the card for the specific chamber type on the vehicle.

Automatic slack adjusters: An ASA should maintain proper adjustment automatically. If an ASA-equipped axle shows excessive stroke, the ASA may be worn or improperly installed. Do not simply re-adjust an ASA and return the vehicle to service — diagnose the root cause.

Air Chamber and Hardware Inspection

Inspect each air chamber housing for dents, cracks, rust-through, or a damaged diaphragm (indicated by air leaking from the exhaust port when brakes are applied). Check the clamp band that secures the two halves of the chamber; it must be tight and free of cracks.

For the brake hardware, check:

Completing the Brake Inspection Report

The inspection report is a legal document. Write legibly, record every measurement, and note every discrepancy. Include the date, vehicle unit number or VIN, odometer, your name, and any OOS conditions found. If the vehicle is safe to return to service, note that. If any OOS condition exists, write it clearly and ensure the vehicle is tagged accordingly before it leaves the shop. Your instructor will compare your recorded measurements against a reference inspection — accuracy within ±1/32 inch on lining thickness is required.

Key terms9

Out-of-service (OOS) criteriaFMCSA-published thresholds that require a commercial vehicle to be taken out of operation immediately until the identified defect is corrected.
Lining thicknessThe remaining depth of friction material on a brake shoe, measured at the thinnest point; compared against OEM minimums and FMCSA OOS limits to determine serviceability.
Slack adjusterA lever arm that connects the air chamber pushrod to the S-cam shaft; its length and adjustment determine how much pushrod travel is needed to apply the brakes.
Pushrod travel (free stroke / applied stroke)The distance the air chamber pushrod moves from the released position to the point of full brake application; excessive travel is an FMCSA out-of-service condition.
Automatic slack adjuster (ASA)A self-adjusting slack adjuster designed to maintain correct pushrod travel as lining wears; if it fails to maintain adjustment, it must be diagnosed and replaced rather than manually re-adjusted.
Spring brake actuatorThe rear chamber of a piggyback air brake assembly that applies the parking and emergency brake using a heavy coil spring; must be caged or removed before working near it because the spring stores lethal energy.
S-camAn S-shaped camshaft turned by the slack adjuster that spreads the brake shoes outward against the drum when the brakes are applied.
Clamp bandThe metal ring that fastens the two halves of an air chamber housing together; must be intact and tight to prevent chamber separation under pressure.
Lockout/tagout (LOTO)A safety procedure that isolates and de-energizes equipment — including exhausting all air pressure — and applies a physical lock and warning tag so the system cannot be re-energized while work is in progress.

Worked examples4

1. You measure the brake lining on the left front steering axle shoe at the thinnest point and get 7/32 inch. The OEM minimum is 1/4 inch (8/32 inch). Is this shoe within OEM spec, and does it meet or exceed the FMCSA OOS threshold?
  1. Convert all values to a common denominator (32nds): measured = 7/32 inch; OEM minimum = 1/4 inch = 8/32 inch; FMCSA OOS threshold for steering axle = 1/4 inch = 8/32 inch.
  2. Compare measured to OEM minimum: 7/32 < 8/32, so the shoe is below OEM minimum and out of OEM spec.
  3. Compare measured to FMCSA OOS threshold: 7/32 < 8/32, so the shoe meets the FMCSA out-of-service condition.
  4. Record on the brake inspection report: Left front, lining = 7/32 inch, OOS — replace lining before returning to service.
Answer: The shoe is below both OEM minimum and the FMCSA OOS threshold. The vehicle must be taken out of service and the lining replaced.
2. On a rear drive axle, you measure three shoes at 9/32 inch, 7/32 inch, and 6/32 inch (all on the same axle). The FMCSA OOS threshold for non-steering axles is 1/4 inch (8/32 inch) when two or more adjacent shoes on the same axle are at or below that limit. How many shoes are at or below the OOS threshold, and is the axle out of service?
  1. List the measurements: 9/32, 7/32, and 6/32 inch.
  2. Identify shoes at or below 8/32 inch (1/4 inch): 7/32 inch — yes; 6/32 inch — yes. That is two shoes.
  3. The FMCSA criterion requires two or more adjacent shoes on the same axle to be at or below 1/4 inch. Both 7/32 and 6/32 shoes are on the same axle. Confirm they are on adjacent positions (same side or adjacent wheel ends as defined by the criteria card).
  4. Two adjacent shoes are at or below the OOS threshold, so the axle meets the OOS condition.
  5. Record each measurement and flag the axle as OOS on the brake inspection report.
Answer: Two shoes (7/32 and 6/32 inch) are at or below 1/4 inch on the same axle, satisfying the FMCSA OOS condition. The vehicle must not be returned to service until lining is replaced.
3. You are checking pushrod travel on a Type 30 air chamber. With brakes released, you mark the pushrod at the chamber face. With a full brake application at system pressure, the pushrod has moved to a point 2-3/16 inches from your mark. The FMCSA OOS limit for a Type 30 chamber is 2 inches. Is this pushrod travel within limit?
  1. Identify the measured travel: 2-3/16 inches.
  2. Convert to a common unit for comparison: 2-3/16 = 2.1875 inches; OOS limit = 2.000 inches.
  3. Compare: 2.1875 inches > 2.000 inches, so the travel exceeds the OOS limit.
  4. Record on the brake inspection report: Type 30 chamber, applied stroke = 2-3/16 inches, OOS — diagnose slack adjuster / lining wear before returning to service.
Answer: Pushrod travel of 2-3/16 inches exceeds the 2-inch FMCSA OOS limit for a Type 30 chamber. The vehicle is out of service until the cause is diagnosed and corrected.
4. A technician measures a brake shoe and reads 11/32 inch on the gauge. They need to record the measurement in decimal inches on the brake inspection report. What is 11/32 inch expressed as a decimal, and how does it compare to the 1/4-inch OEM minimum?
  1. Convert 11/32 to decimal: 11 ÷ 32 = 0.34375 inch.
  2. Convert OEM minimum 1/4 inch to decimal: 1 ÷ 4 = 0.250 inch.
  3. Compare: 0.34375 > 0.250, so the lining is above the OEM minimum.
  4. Record: lining = 0.344 inch (rounded to nearest thousandth), within OEM specification.
Answer: 11/32 inch = approximately 0.344 inch, which exceeds the 0.250-inch (1/4-inch) OEM minimum. The shoe is serviceable.
In class: With vehicle on approved stands and air system exhausted/LOTO applied, remove all brake drums or access inspection holes, measure lining thickness at the thinnest point on each shoe, compare to OEM minimum and FMCSA out-of-service criteria, and record measurements on the brake inspection report. · Check each slack adjuster for correct free stroke (FMCSA pushrod travel limit) using a ruler at the clevis; identify any slack adjuster that exceeds the limit and note required corrective action; check automatic slack adjuster operation by applying the brakes and observing travel. · Inspect air chambers for corrosion, damage, and clamp band tightness; inspect all brake hardware (return springs, anchor pins, cam rollers) for wear; inspect air lines at the axle for chafing or leaks; compile all findings in the brake inspection report and present to instructor.
Materials: Class 8 truck on jack stands, brake-lining gauge, ruler, FMCSA out-of-service criteria card, brake inspection report form, air supply, LOTO kit, safety stands
Check: Instructor reviews completed brake inspection report against actual vehicle condition: all lining measurements recorded within ± 1/32 inch of instructor reference, all slack adjuster strokes measured and compared correctly; any undetected out-of-service condition = mandatory remediation and re-inspection.

Practice check8

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

Safety / Shop Practices
1. Before removing brake drums on a spring-brake-equipped axle, the MOST important safety step is to:
Brakes (ASE T4 / FMCSA Regulations)
2. According to FMCSA out-of-service criteria, a steering axle brake shoe is out of service when lining thickness at the thinnest point is less than:
Brakes (ASE T4)
3. A technician measures an air chamber pushrod and finds applied stroke exceeds the FMCSA limit for that chamber type. The axle is equipped with automatic slack adjusters. The correct action is to:
Brakes / Math Skills (ASE T4)
4. You are measuring lining thickness and obtain a reading of 6/32 inch on a non-steering axle shoe. Expressed in decimal inches, this measurement is:
Brakes (ASE T4)
5. When inspecting brake return springs during a PM inspection, a broken return spring is a concern primarily because it will:
Brakes / Preventive Maintenance (ASE T4)
6. During a brake PM, you notice an oily residue on the outside of an air line fitting at the rear axle. This MOST likely indicates:
Preventive Maintenance Documentation (ASE T8)
7. The brake inspection report completed during a PM inspection is best described as:
Brakes (ASE T4)
8. A cam roller that shows a flat spot on its rolling surface will MOST directly cause:
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