Vehicles built in the last 20 years are required to have self-adjusting brake adjusters (S-ABAs). But even though they’ve been around awhile, some people don’t seem to know how S-ABAs work. Here’s proof: according to one survey conducted by the Commercial Vehicle Safety Alliance, 53 percent of drivers said that automatic slack adjusters never go out of adjustment. That’s not true, and it’s a potentially dangerous misconception.

In fact, automatic slack adjusters aren’t always perfect and do go out of adjustment over time with use. Such changes are not always noticeable during day-to-day driving, and are typically a symptom of a problem with a foundation component, adjuster or drum. And out-of-adjustment brakes can have significant implications for your stopping power and your ability to meet federal reduced stopping distance (RSD) regulations. That’s why S-ABAs should be checked as part of every pre-trip inspection.

If your S-ABAs are out of adjustment, avoid the temptation to make a manual adjustment. Such shortcuts usually fail after a few brake applications and can give drivers a false sense of security. In addition, brake stroke is the most ticketed item in DOT inspections. Inspectors will measure brake chamber pushrod stroke down to 1/8 of an inch, so don’t take chances.

The good news is that it’s fairly easy to measure pushrod stroke. All it takes is a little time, someone to help, and a few supplies: chalk to mark the pushrods, eye protection, and a ruler, flashlight, pencil and paper. Then follow these four steps:

  1. Park your vehicle in a safe place and chock the wheels. With the engine running, release the spring brake and confirm you have 90-100 psi supply pressure in the air brake system reservoir. Then shut off your truck’s engine.
  2. Visibly inspect each brake to confirm it’s in its normal release position and there are no visible problems. As you do so, mark each pushrod with chalk to establish a reference starting location for where the pushrod exits the brake chamber.
  3. Push the brake pedal down until it stops, and don’t let it up. Ask your assistant to measure and record the distance each pushrod has stroked. If applying the brake multiple times causes the pressure reading to drop below 90 psi, take a break from testing so it can rebuild to 90-100 psi, and then test again.
  4. Compare the pushrod stroke numbers with the regulation stroke limits for your brake chamber. If any measurements are near or at the regulation stroke limits, it’s time to have your brake chamber, drums, S-ABAs and foundation components inspected and serviced.

If anything seems off with your slack adjusters, turn to the maintenance team at your local Volvo Trucks dealership to check out your brake system and provide the latest information on stopping adjustment regulations and stroke limits. In addition, you can find all the lubricants and repair parts you need for optimal brake performance by visiting the SELECT Part Store™. When you enroll, you’ll have 24/7 access to our online ordering system, timely delivery and member-only discounts.

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