Preventive Maintenance Tips for Foundation Drum and Air Disc Brakes

Taking the time to focus on a few key details when performing preventive maintenance of a vehicle’s brakes can elevate the effectiveness of a routine brake job and have an impact on brake life, performance and safety.

Content Dam Up En Articles Print Volume 22 Issue 2 Product Focus Vehicles Accessories Preventive Maintenance Tips For Foundation Drum And Air Disc Brakes Leftcolumn Article Thumbnailimage File

By Keith McComsey

It’s easy to view preventive maintenance of a vehicle’s brakes—whether they’re foundation drums or air discs—as little more than a quick check, a grease job, and some new brake pads before getting things rolling again. But taking the time to focus on a few key details can elevate the effectiveness of a routine brake job and make an impact on brake life, performance and safety.

Drum Brakes—Free Play and Brake Stroke

S-cam drum brakes are the most common air foundation brakes on the road—and a familiar sight to every wheel-end technician in the North American trucking industry. At the heart of keeping them operating safely and effectively is maintenance of the correct free play and brake stroke.

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Automatic slack adjusters (ASAs) should not be manually adjusted. While there are many factors that can cause an ASA to overstroke, none will be remedied by a manual readjustment.

One method of measuring a brake’s free play and stroke is by cutting a short segment of tape measure and attaching a magnet to one end. By placing the tape measure against the face of the brake chamber, it can be used to measure the travel of the pushrod at the center of the large clevis pin. Move the slack adjuster using a tool such as a small pry bar, and check how far the center of the large clevis pin moves before the brake shoes come in contact with the drum. This free play should be between 3/8 inch and 5/8 inch, and consistent across all brakes. If the free play falls outside that range, all brake components should be examined for damage or out-of-spec conditions.

Measure brake stroke by bleeding the air system down to between 90 psi and 100 psi and mark the center of the clevis pin’s position on the tape measure. Next, have someone fully apply the brakes and hold the pedal down, or wedge the brake pedal in the fully applied position to actuate the brakes between 90 psi and 100 psi. Note the new position of the center of the clevis pin on the tape measure: The distance between the two marks indicates the brake’s power stroke. The maximum allowable power stroke varies depending on the chamber size and type.

Slack Adjusters and Chambers

Despite their commonality, drum brakes are still the subject of a long-standing misconception regarding automatic slack adjusters (ASA). If a brake is out of adjustment, Bendix stresses that the ASA should not be manually adjusted. While there are many factors that can cause an automatic slack adjuster to overstroke, none will be remedied by a manual readjustment. Additionally, manually adjusting an ASA involves working against its internal back-off clutch, which can impact the component’s life. Always refer to the manufacturer’s instructions and mechanism tests to determine whether the slack adjuster is properly adjusting the brake.

Another key piece of information on drum brake chambers and slack adjusters is that there are two common types of brake chambers—standard and long stroke—as well as two common sizes of slack adjusters (5.5 inch and 6 inch, as measured from the center of the camshaft to the center of the clevis pin). For both regulatory and brake performance reasons, all slack adjusters and air chambers on any given axle must be the same type and size.

Greasing and More

It’s always important to grease brakes to capacity to prevent condensation from forming inside the brake components, which can cause rust and corrosion. Seals inside the cam tube are engineered to let excess grease out of the area, which means new grease should be added until any dirty grease or condensation is visibly purged out of the end of the cam tube near the slack adjuster. If grease is purging from the opposite end of the cam tube, near the S-cam head, it indicates a bad grease seal that needs to be replaced.

When it’s time for a brake job and new friction, it’s also time to check the cam bushings and shims. After removing the wheel and brake drums, disconnect the slack adjuster from the chamber clevis, remove the dial indicator from the inboard end of the cam, and rotate it over to the cam head. Next, manually move the cam head up and down to measure the radial play, which should be less than 0.030 inch. If it’s at or beyond that threshold, either the cam bushing or camshaft is worn and will need to be replaced; too much radial play leads to a loss of stopping power as well as uneven lining wear. Because cam bushings are a low-cost component that can make a large difference in the long run, Bendix recommends keeping radial play below 0.010 inch by replacing them as needed. Bronze bushings are recommended for severe-duty vehicles and heavy-wear brake situations, while nylon bushings generally suffice for line haul and highway applications.

After putting the slack adjuster back on, place a washer between the adjuster and the cam tube, and another on the cam head to help keep dirt and contaminants out of the bushings and seals. Add or remove washers (shims) between the slack and the end of the cam tube to ensure it lines up directly with the center of the clevis. The shims on the outside of the slack adjuster next to the snap ring are for adjusting “end play,” which should be between .005 inch and .025 inch on the dial indicator. End play will increase as the brake is worn in, so the closer it is to zero, the better. Because shims are worn down through exposure, Bendix recommends changing shim packs during brake jobs. The few minutes spent making these measurements, re-shimming and replacing bushings will pay off in the extension of brake life and performance.

Disc Differences

Although the benefits of air disc brakes (ADB) include extended service intervals when compared to drum brakes, ADB should still be checked during every preventive maintenance inspection. This will contribute to the brakes’ longevity and generally doesn’t require wheel removal.

Begin with a visual check of caliper movement to ensure it slides freely, and look at the rotor as well, with an eye out for surface cracks beyond allowable limits. Use an inspection mirror for the best view of the rotor from outside to inside. The mirror is also useful for checking pad wear; the minimum allowable friction material thickness is 2mm.

Next, make sure to check the mounting hardware of calipers and air chambers; loose mounting bolts will get a brake placed out-of-service (OOS) in a roadside inspection. Ensure the guide pin covers are not missing, and check for proper rotor/pad running clearances.

If these inspections raise any red flags, such as a caliper that doesn’t move freely, large rotor cracks, or pads wearing unequally from the inboard pad to the outboard pad, a more thorough check should be conducted with the wheel off. In this case, also look for damaged tappets and guide pin boots. These routine checks act as early warnings for issues that require immediate attention, while helping protect the life of air disc brake components.

Today’s advanced drum and air disc brakes are engineered to deliver better performance and longer life, and to support advanced safety technologies from full stability to collision mitigation. The right maintenance practices will keep them operating at their best, lowering fleets’ operating costs and helping make the roads safer for everyone. UP

About Bendix Spicer Foundation Brake LLC:
Bendix Spicer Foundation Brake LLC combines and expands the complementary wheel-end foundation brake technologies of Bendix Commercial Vehicle Systems LLC and Dana Commercial Vehicle Products LLC. The joint venture, formed in July 2004, is a single, complete source for OEM brake system design, manufacturing, hardware and support for all foundation brake components and actuation systems, as well as all-makes coverage of some 50,000 medium-duty and heavy-duty aftermarket parts. Please visit

About the author: Keith McComsey is director of Marketing and Customer Solutions at Bendix Spicer Foundation Brake LLC. He has been involved in the commercial trucking industry for 25 years in engineering and product management roles at both the truck OE and supplier levels. McComsey works closely with customers regarding current and future Bendix brake technologies.

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