In 30% of truck crashes, air brake problems are a “significant” factor.
One of the most common air brake issues is reduced brake force, due to increased pushrod stroke. It can lengthen over time, simply from typical use.
Air brake adjustment fixes this. It's the act of resetting the distances and positions of components within an air brake system: optimizing for operational clearance and, critically, bringing the pushrod stroke length back within regulatory limits.
Yet, air brake adjustment isn't exactly easy. And, since doing it incorrectly can pose serious risks, laws regulate not only the air brake system itself, but the adjustment process.
Navigating this aspect of heavy duty truck maintenance isn't necessarily straightforward. In this guide, delve into the parts and processes involved in air brake adjustment. Then, learn the gold standard air brake adjustment procedure, broken down into clear, actionable steps.
How Air Brake Adjustment Keeps Vehicles Safer
To understand how air brake adjustment improves road safety, it helps to keep in mind how air brakes work.
When a driver hits the brakes, the brake caliper (or S cam) hits the rotating brake disk / drum, applying kinetic friction—a powerful stopping force—to halt the vehicle.
In an air brake system, pressing the brake pedal pressurizes the brake chamber, extending the pushrod to activate the caliper / S cam.
“Pushrod stroke” is the full length the pushrod travels when the bake chamber is fully pressurized. That is, it's a metric that shows how far the rod has to go, in inches or millimeters, and how much braking it takes to get it there.
As it travels, the pushrod loses power. The longer the pushrod stroke is, the weaker the stopping force—and, thus, the longer it takes for a vehicle to stop after braking.
Long stopping distances are dangerous.
Adjustment Optimizes For Stopping Power
With regular adjustment, a truck’s stopping distance stays within a safe limit.
A table demarcating the legal maximum stopping distances for trucks, by truck weight and speed, is published by the FMCSA here: Table II—Stopping Distance in Feet.
Moreover, correct adjustment keeps brakes lasting longer. If the brake’s caliper or S cam hits the rotating piece with intense force, too often, it wears down the pad faster. Adjustment makes it far easier to stop with less force.
The trouble is, manually adjusting air brakes takes 10-30 minutes, and it isn’t intuitive. Air brakes’ adjustment should be inspected every trip, if not daily.
On average, one or more air brakes requires adjustment every 10,000-12,000 miles or so, and it’s not uncommon for an over-the-road freighter to travel 100,000 miles in a year. Likewise, heavy-duty trucks driving fewer miles—like sanitation vehicles or end-dump trucks—utilize brakes more frequently in the course of their work.
In both cases, a lot of time ends up being spent on brake adjustment.
Automatic brake adjusters change that.
Automatic Brake Slack Adjusters Streamline Safety
Engineers had tried to tackle the issue for decades.
The first automatic brake adjustment machines were invented in the 1940s. But, the first truly viable model was made in 1968. That year, the company Haldex adapted automatic railway air brake regulators for trucks, and brought them to the market.
Automatic brake adjusters—also called automatic slick adjusters (ASAs)—use an adjuster lever balanced at an optimal gap between the brake drum and S cam’s shoe. As the gap widens, the lever is unbalanced, tipping to pull the automatic adjuster’s gear forward by one tooth.
This, in turn, rotates a threaded adjuster, shifting the S cam’s position. The new position reduces the gap, compensating for the wearing of the cam’s brake show and re-balancing the adjuster lever.
Over time, engineers iterated ASAs to be compatible with disc brakes as well.
By the early 1990s, ASA devices were ubiquitous. In 1992, The National Transportation Safety Board (NTSB) ran an investigation of truck accidents, and found improperly adjusted air brakes were a significant, causative factor in 46% of the crashes. Those with automatic adjusters were far less likely to be involved in a crash.
In 1994, Congress passed regulations mandating all U.S. trucks be built with Automatic Break Adjusters moving forward. Canada followed suit soon after.
Airbrake Adjustment In Twenty-First Century
Today, industry pros estimate 95%-97% of all trailers for heavy duty trucks—and other commercial vehicles—are equipped with ASAs. That said, fewer semi trailers have automatic brake adjusters on their wheels, as more trailers 30+ years old are still usable.
For modern drivers and fleet managers alike, mastering air brake adjustment is as vital as ever.
Which Laws Regulate Air Brake Adjustment?
In the North American Trucking industry, most air brake regulations are laid out in Title 49 of the CFR, and The Official Air Brake Handbook in Canada. Here's what these regulations mean in practice.
Title 49 of the Code of Federal Regulations (CFR)
The FMCSA and DOT mandates regulating commercial trucks’ air brake safety are codified in 49 CFR, Standard No. 121 § 571.121.
The law is incredibly detailed. While the complexities are important for industry professionals to understand, summaries are also useful.
Most FMCSA- and DOT-approved summaries are published by the nonprofit Commercial Vehicle Safety Alliance (CVSA). For example, the pamphlet “Air Brake Pushrod Stroke: Why Is It So Important?” offers a simplified, clear depiction of the standard air brake adjustment procedures mandated by law in the U.S.
To get a clearer picture of optimal air brake readjustment metrics at a glance, check out the limit tables in Title 49:
Canada Regulates Air Brake Adjustment by Province
The Ministry of Transportation of Ontario, Canada, publishes its routinely updated regulations in The Official Air Brake Handbook.
“When brakes with manual slack adjusters are out of adjustment, they must be re-adjusted so that the pushrod stroke is less than the adjustment limit. Only certified technicians may repair and perform brake re-adjustments on manual slack adjusters. In Ontario, drivers can become certified to perform brake re-adjustments on manual slack adjusters. Only those who have obtained certification are permitted to perform brake re-adjustments. This certification does not permit you to manually re-adjust automatic slack adjusters.
Automatic slack adjusters, if they are working properly, do not require regular re-adjustment. If periodic re-adjustments are required, it means that the automatic slack adjuster is defective and must be repaired by a certified technician. Only certified technicians are permitted to perform re-adjustments or repairs on automatic slack adjustors.”
Fines & Penalties For Poor Air Brake Adjustment
In the U.S., a truck driver or fleet manager is not typically fined for having improperly adjusted brakes. However, substandard brake performance does result in an Out-Of-Service order placement on the truck.
It is unlawful to drive a truck declared Out-Of-Service by the DoT, until the order is officially removed.
Violating an Out-Of-Service order can absolutely get a driver or fleet manager fined. When an OOS truck or HDV with maladjusted air brakes is used, it's likely one of two violations is being committed:
“Violation—Operation of a commercial motor vehicle or intermodal equipment [including air brakes] by a driver after the vehicle or intermodal equipment was placed out-of-service and before the required repairs are made.
Penalty—$2,364 each time the vehicle or intermodal equipment is so operated.
Violation—Requiring or permitting the operation of a commercial motor vehicle or intermodal equipment placed out-of-service before the required repairs are made.
Penalty—Up to $23,647 each time the vehicle or intermodal equipment is so operated after notice of the defect is received.
(This violation applies to intermodal equipment providers and motor carriers, including an independent owner operator who is not a “driver,” as defined in IV(a) above.)”
These fines are standard penalties established in 49 CFR, Standard No. 121.
Best Practices: Adjuster Type, Steps & Maintenance
Regardless of adjuster type, it's important to check your air brakes before every trip, as part of your routine pre-trip inspection. Many industry leaders recommend going beyond that, and checking your air brakes every day.
Beyond that, though, air brake adjustment best practices vary by type. Here are a few things to know.
Manual Slack Adjuster Vs. Automatic Slack Adjuster
Manual slack adjusters aren't particular components. Instead, they're tools to adjust air brakes manually if a truck doesn't have an ASA.
The average heavy-duty truck without ASAs requires manual adjustment about once per month. To legally adjust air brakes manually in the U.S., a person must have appropriate certification and training.
How To Manually Adjust Air Brakes In 3 Steps
Please note that these steps should not replace training and certification from a qualified automotive technician.
Setup the truck so you can safely adjust it. Secure the wheels with chocks, then release the parking brakes.
Measure the pushrod stroke to determine the appropriate degree of adjustment. For more insights into this process, read “Step-by-step procedure for measuring applied pushrod stroke,” from The Official Airbrake Handbook.
Tighten the adjuster nut until the pads of your air brakes’ S cam (or calipers) press against the surface of the brake drum (or disc). Then, back up by no more than ¼ of a turn.
Adjustment can typically be done immediately post-inspection. CVSA offers in-depth guidance and instructions through its Operation Airbrake program and publications.
Automatic Slack Adjuster Maintenance
Automatic slack adjusters are mandatory parts of trucks built after 1994. Fortunately, as their name implies, ASAs don’t require manual adjustment.
However, they should still be checked regularly. If an air brake is maladjusted despite having an automatic brake adjuster, it’s time for a trained, certified technician to intervene. Do not “adjust” an ASA independently.
Preventative Truck Brake Maintenance Best Practices
Regular maintenance is key to keeping every part of a truck running smoothly. Air brakes are no different. For in-depth insights into air brake care and service, explore “Your Complete Guide To Air Brake Troubleshooting,” a comprehensive resource by Point Spring’s heavy-duty truck specialists.
How To Build A Culture Of Air Brake Safety
Brake safety continues to evolve. It’s smart for a fleet’s culture to evolve with it. In 2022, a study demonstrated that new automatic braking technology cuts the rate of crashes by 40%. Yet, just as with ASAs in previous decades, there are barriers to adoption.
Fleet managers play a vital role in shaping the culture of the trucking industry. To elevate air brake safety as an industry-wide priority, fleet leaders can:
Offer or require paid brake adjustment training for drivers
Emphasize the importance than certified brake technicians perform inspections and repairs
Institute onsite inspections by certified technicians
Fund or participate in a streamlined fleet brake maintenance program
Encourage and fund ongoing air brake compliance training for onsite or partner automotive techs
Safe, Swift Air Brake Adjustment At Point Spring
All Point Spring & Driveshaft Co. technicians are certified in manual air brake adjustment and ASA repair. We continually update training, so our heavy-duty truck service is never less than the best.
What can we do for you? Call us at +14122646714, or contact us online today to learn more about our services.