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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Air Brakes: Systems, Service, and Compliance › Lesson 5
TRN-DIES20-03 · Lesson 5 · 20 clock hours · lab

Foundation Brakes: S-Cam Drum Brake Components and Operation

Objective. Students will disassemble an S-cam foundation brake assembly, measure drum diameter, lining thickness, and cam-bore wear, and determine serviceability against published specifications.

⚠ 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.

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Foundation Brakes: S-Cam Drum Brake Components and Operation

What Is a Foundation Brake?

The foundation brake is the mechanical assembly at each wheel that actually stops the vehicle. On most commercial trucks and trailers, the standard foundation brake is the S-cam drum brake. Understanding how each component works—and how to evaluate its condition—is a core skill for any diesel and heavy truck technician.

Major Components

Brake drum: A cast-iron cylinder that rotates with the wheel hub. When the brakes are applied, the brake shoes press outward against the drum's inner friction surface, converting kinetic energy to heat. The drum must be within its manufacturer-specified diameter range to provide safe, consistent braking. Oversize drums (worn beyond the discard diameter) cannot absorb heat properly and may crack.

Brake shoes and lining: Two curved shoes, each faced with friction lining, are anchored at one end on a fixed pivot (anchor pin) and spread apart at the other end by the S-cam. The lining wears gradually with each stop. Minimum lining thickness is specified by both FMCSA regulations and the OEM; when lining is at or below minimum, the shoe must be replaced. Contaminated lining—from grease or brake fluid—must also be replaced regardless of thickness.

S-cam: The heart of the actuating mechanism. The shaft is forged with an S-shaped cam profile at the inner end. When the slack adjuster rotates the cam shaft, the S-cam lobes push the roller ends of both brake shoes outward simultaneously, pressing lining against drum. The cam profile ensures a smooth, progressive force application. The cam rides in bronze or nylon cam bushings pressed into the spider (backing plate); worn bushings allow the cam to shift off-center, causing uneven lining contact and brake fade.

Slack adjuster: A lever arm splined to the S-cam shaft. The air chamber pushrod pushes the slack adjuster, rotating the cam. Automatic slack adjusters (ASAs) maintain a consistent pushrod travel as lining wears; manual adjusters must be checked and turned periodically. Slack adjuster length and chamber size together determine brake torque.

Return springs and hardware: Heavy coil springs retract the shoes when air pressure releases. Retaining hardware (hold-down clips, anchor pins, roller kits) keeps the shoes positioned correctly. Weak or broken springs allow shoe drag, accelerating wear and causing heat buildup.

Anchor pin: The fixed pivot at the heel of each shoe. Worn anchor pins allow shoe movement and reduce braking consistency.

How the S-Cam Brake Operates

When the driver applies the brakes, compressed air enters the brake chamber, extending the pushrod. The pushrod rotates the slack adjuster arm, which turns the S-cam shaft. The S-shaped lobes force the shoe rollers apart, pushing the lining against the rotating drum. Friction between lining and drum slows the wheel. On release, the return springs pull the shoes back, the cam reverses slightly, and the pushrod retracts.

Disassembly Sequence and Safety

Before starting: Chock the vehicle or training axle securely. Apply lockout/tagout (LOTO) procedures if the axle is mounted on a powered stand. Wear safety glasses, chemical-resistant gloves, and a NIOSH-approved dust respirator at all times. Brake dust from older linings may contain hazardous materials; never use compressed air to blow dust—use a brake wash station or vacuum system with a HEPA filter.

Photograph each stage before removal. Work in this order: remove the wheel, then the hub/drum, then the shoes and hardware, then the S-cam and bushings. Keep all hardware organized by position (leading/trailing shoe) for inspection.

Dimensional Inspection

Drum diameter: Use an inside brake drum micrometer, measuring at several locations across the friction surface. Compare to the drum's maximum allowable diameter (the discard diameter) cast or stamped on the drum. A drum worn beyond discard diameter must be replaced. A drum within spec but showing scoring may be reground if sufficient material remains after machining.

Lining thickness: Use a lining thickness gauge at the center and ends of each shoe. FMCSA sets a regulatory minimum for in-service vehicles; OEM specifications may be more conservative. Replace any shoe at or below the minimum, and always replace shoes in axle sets (both sides) to maintain equal braking force side to side.

Cam-bore diameter: Use a telescoping bore gauge or small-hole gauge, then measure with an outside micrometer. Compare to the OEM specification for cam bushing inside diameter. Excessive bore wear allows lateral play in the cam shaft, reducing braking effectiveness and accelerating lining and drum wear.

Serviceability Calls

For each component, make one of three decisions:

Document your reasoning on the lab data sheet. A parts decision without a measurement to support it is not acceptable.

Reassembly Reminder

When reassembling, lubricate cam bushings, anchor pins, and shoe contact pads with high-temperature brake lubricant only—never grease lining contact surfaces. Torque all fasteners to OEM specification. After reassembly, adjust brakes and verify pushrod travel is within the legal limit before the axle is returned to service.

Key terms9

S-camAn S-shaped camshaft component that, when rotated by the slack adjuster, forces both brake shoes outward against the drum to create braking force.
Brake drum discard diameterThe maximum allowable inside diameter of a brake drum, stamped on the drum by the manufacturer; a drum worn to or beyond this measurement must be replaced.
Lining thicknessThe measured depth of friction material remaining on a brake shoe; when at or below the minimum specification, the shoe must be replaced.
Cam bushingA bronze or nylon sleeve pressed into the brake spider that supports the S-cam shaft; wear in the bushing allows cam lateral movement and uneven brake application.
Slack adjusterA lever arm splined to the S-cam shaft that converts linear pushrod movement from the air chamber into rotational force on the cam; may be manual or automatic.
Anchor pinThe fixed pivot pin at the heel (toe) of each brake shoe that holds the shoe in position and acts as the reaction point when the cam applies force.
Brake spider (backing plate)The stationary steel plate bolted to the axle flange that supports all foundation brake components including the cam bushings, anchor pins, and return spring brackets.
Inside brake drum micrometerA specialized measuring tool designed to accurately measure the inside diameter of a brake drum at multiple points to check for wear and out-of-round condition.
LOTO (Lockout/Tagout)An OSHA-required energy-control procedure in which equipment energy sources are isolated and locked in the off position before service work begins to prevent unexpected movement or activation.

Worked examples4

1. A technician measures a brake drum at three positions across the friction surface and records: 16.480 in., 16.492 in., and 16.475 in. The drum's stamped discard diameter is 16.500 in. Is the drum serviceable?
  1. Identify the discard diameter from the drum: 16.500 in.
  2. Compare each measurement to the discard diameter: 16.480 < 16.500 ✓; 16.492 < 16.500 ✓; 16.475 < 16.500 ✓.
  3. Check for out-of-round: largest reading minus smallest = 16.492 − 16.475 = 0.017 in. Note this value; consult the OEM spec for maximum allowable out-of-round (commonly 0.010–0.015 in. depending on OEM).
  4. All measurements are below the discard diameter, so the drum is not worn out by diameter. However, the out-of-round of 0.017 in. may exceed the OEM limit—flag for possible resurfacing and recheck after machining.
Answer: The drum is within discard diameter at all three measurement points and may be reused or resurfaced, but out-of-round should be compared to the OEM limit before a final serviceability call is made.
2. A student measures lining thickness at the center of a brake shoe and records 7/32 in. (0.219 in.). The OEM minimum lining thickness is 1/4 in. (0.250 in.). What is the correct parts decision?
  1. Convert measurements to a common unit (decimal inches): measured thickness = 0.219 in.; minimum spec = 0.250 in.
  2. Compare: 0.219 in. < 0.250 in. — the lining is below the minimum specification.
  3. Determine action: any lining at or below minimum must be replaced.
  4. Remember: replace both shoes on the same axle end together to maintain balanced braking.
Answer: Replace the brake shoe. The lining at 0.219 in. is below the 0.250 in. minimum; replacement of both shoes on the axle set is required.
3. A bore gauge reading transferred to an outside micrometer shows the cam bushing bore is 1.628 in. The OEM specification for cam bushing inside diameter is 1.595 in. to 1.620 in. What is the serviceability call, and by how much does the bore exceed the specification?
  1. Identify the OEM maximum allowable bore diameter: 1.620 in.
  2. Compare measured bore to maximum: 1.628 in. > 1.620 in. — bore is oversize.
  3. Calculate the amount of excess wear: 1.628 − 1.620 = 0.008 in. beyond the maximum limit.
  4. Because the bore exceeds the maximum specified diameter, the cam bushing must be replaced before the brake is returned to service.
Answer: Replace the cam bushing. The bore is 0.008 in. oversize beyond the OEM maximum of 1.620 in., indicating excessive wear that will allow cam shaft lateral movement.
4. A drum currently measures 16.440 in. inside diameter. The OEM discard diameter is 16.500 in. A resurfacing cut will remove 0.030 in. from the diameter (0.015 in. per side). What will the drum diameter be after resurfacing, and is resurfacing acceptable?
  1. Calculate post-cut diameter: 16.440 + 0.030 = 16.470 in.
  2. Compare post-cut diameter to discard diameter: 16.470 in. < 16.500 in.
  3. Because the drum will still be below the discard diameter after the cut, resurfacing is acceptable.
Answer: After resurfacing, the drum will measure 16.470 in., which is below the 16.500 in. discard diameter. Resurfacing is acceptable.
In class: Complete brake disassembly on a training axle: remove wheel, hub, drum, shoes, and hardware; students photograph each stage for their lab log before removing the next component. · Dimensional inspection: each student measures drum diameter with an inside brake drum micrometer, lining thickness with a lining gauge, and cam-bore diameter with a bore gauge, recording all measurements on a lab data sheet. · Serviceability determination: students compare their measurements to the OEM specification table and write a parts decision (reuse, regrind, or replace) with justification for each component.
Materials: Training axle with S-cam brake assembly, inside brake drum micrometer, lining thickness gauge, bore gauge set, torque wrench, brake spoon, PPE (gloves, safety glasses, dust respirator), OEM brake service manual, lab data sheets, camera or smartphone
Check: Lab data sheet with all measurements recorded and serviceability calls correct within ±0.005 in. of instructor measurement; 90% measurement accuracy required

Practice check8

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

ASE T4 Brakes – Foundation Brake Systems
1. Which component rotates to force both brake shoes outward against the drum when the air chamber extends?
ASE T4 Brakes – Foundation Brake Systems
2. A brake drum is measured and found to be 0.003 in. below its stamped discard diameter. What is the correct action?
ASE T4 Brakes – Safety and Hazardous Materials
3. Why must brake dust never be removed with compressed air?
ASE T4 Brakes – Foundation Brake Systems
4. A student measures lining thickness at 0.245 in. and the OEM minimum is 0.250 in. Which of the following actions is correct?
ASE T4 Brakes – Foundation Brake Systems
5. Worn cam bushings in an S-cam brake assembly most directly cause which condition?
ASE T4 Brakes – Safety and Shop Practices
6. Before disassembling a brake assembly on a powered training stand, which safety procedure is required by OSHA to prevent unexpected movement?
ASE T4 Brakes – Dimensional Inspection and Serviceability
7. A drum currently measures 16.455 in. A resurfacing cut will add 0.040 in. to the diameter. The discard diameter is 16.500 in. What will the post-cut diameter be, and is resurfacing acceptable?
ASE T4 Brakes – Foundation Brake Reassembly and Service
8. Which lubricant application during brake reassembly is correct?
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