VIDEO: How drivers can ward off brake risk and inspection hassles

By Larry StewartSeptember 20, 2022

Maximum allowable pushrod stroke measured using the applied-stroke method -- what DOT inspectors use -- is 2 inches. Image: CDL Air Brakes SMART

Nearly 38% of vehicle-out-of-service conditions found at commercial motor vehicle inspections in North America concerned the condition of brake systems. Federal and provincial laws require that air-brake adjustment be checked daily, during the pre–trip inspection. Your company may have maintenance people to

keep vehicles running safely, but the driver can be fined if the brakes on the vehicle you are driving are found to be out of adjustment.

Over the three days in May of this year, the Commercial Vehicle Safety Alliance’s (CVSA) International Roadcheck inspection and enforcement initiative, CVSA-certified inspectors conducted 59,026 inspections and issued out-of-service (OOS) orders on 23% of those vehicles. (get the North American Standard Out-of-Service Criteria)

“Brake systems” were responsible for the most-frequent OOS violations at 25.2% of the total and “Defective service brakes,” a separate category, was responsible for another 12.7% of violations.

The Commercial Vehicle Safety Alliance 2022 inspection event placed 23% of vehicles inspected across North America out of service, predominantly because of brake issues. Table: Commercial Vehicle Safety Alliance

“The reality is that brake failures and runaways that result in crashes are almost never caused by an air system failure, but by the absence of routine brake maintenance, or by the driver failing to check brakes on a daily basis,” according to a brake-adjustment recommendation by Palmer Leasing. “You’re also required by law to check your brakes before driving down steep grades that are posted with regulatory signs.”

How to measure pushrod stroke

With a prying tool, something to mark the pushrod and a flashlight, a single person can check service brake adjustment (don’t forget eye protection and bump cap):

  1. Make sure air-system pressure is at least 90 to 100 psi and all parking brakes are released
  2. Mark the pushrod where it exits the brake chamber
  3. Pull the pushrod out from the brake chamber, using a tool for leverage
  4. Measure the distance between the chalk mark and the face of the brake chamber

Palmer Leasing suggests half to three-quarters of an inch of free stroke is a good range for a type 30 standard stroke brake chamber activated using the prying method. If the distance is more than three-quarters of an inch, the brakes need adjustment.

Another way to measure brake stroke is for a second person to step on the brakes hard with the system at 90 to 100 psi while you measure pushrod stroke. Suppose your pre-trip inspection shows one or more of your brake-chamber pushrods is stroking more than 2 inches. Two inches is the limit for this applied-stroke measurement. When the brake drum is cool, the brake may seem to be effective in light braking, so it is easy to be lulled into a false sense of security.

The risk in maladjusted brakes

Cast iron brake drums expand when heated, requiring the air chamber to stroke further as the temperature rises. On a long downhill grade or an unforeseen event requiring the driver to make a sudden stop, the brake chamber could bottom out, severely limiting braking power or even causing a runaway.

A DOT inspection can take a truck out of service if there’s more than 2 inches of brake pushrod travel.

Confirm auto slacks are working

Automatic slack adjusters are supposed to maintain a safe distance between brake drums and brake pads as the pads wear. Working slack adjusters bring the shoes as close to the drums as possible. Assuming the drum and pads and rest of the foundation brake are in good working condition, that will limit pushrod travel to well less than 2 inches. More importantly, slack adjusters will minimize the amount of pad and pushrod travel for even hard braking. It gives you faster brake response and reduces the volume of air used to apply the brakes.

If the adjusters don’t get greased regularly, their ratcheting mechanism will jam up. Here’s what you can do as the first line of defense against mal-adjusted service brakes.

  1. Chock the wheels
  2. Grease the slack adjusters
  3. Open the control valves for the tractor spring parking brake and trailer-brake air supply
  4. Start the truck
  5. When the air system is above 90 psi, make a full brake application and release
  6. Repeat these hard brake applications with 90 to 100 psi in the brake system five more times

The hard brake applications are intended to get the ratchets in automatic slack adjusters to rotate to the next pawl.

Recheck pushrod stroke. If it’s less than 2 inches, your six hard brake applications got the slack adjusters working again. If there’s still too much pushrod stroke, it’s time to have a qualified brake service technician look at those foundation brakes.

We’re talking about brake maintenance, so liability is a significant consideration. If your truck is involved in an accident after you’ve been wrenching on the slack adjusters, things could go badly for you in court if you’re not a brake tech.

But drivers remain responsible for knowing when to bring the brake pros into the picture. Other elements of your daily brake inspection include checking for wear and excessive play on other brake components including:

  • S-cam bushing wear
  • S-cam spline wear
  • Slack adjuster play (worn gear)
  • Clevis pin wear
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