How to Adjust Air Brakes on a Truck: A Step-by-Step Guide for Beginners

Working on air brakes can seem intimidating, but learning how to properly adjust them is an essential skill for truck owners and mechanics. With the right techniques and safety precautions, you can easily inspect and tune up your truck’s air brakes to keep them performing smoothly. This comprehensive guide walks through the entire process in simple, easy-to-follow steps.

An Introduction to Air Brakes

Before diving into the adjustment process, let’s first understand what air brakes are and how they work.

Air brakes use compressed air to actuate the brake pads, creating friction that slows the vehicle. When you hit the brake pedal, that opens a valve that releases pressurized air into the brake chamber. The expanding air pushes the brake pads outward onto the brake drum, slowing your truck.

Compared to hydraulic brakes, air brakes provide superior stopping power for heavy vehicles. They also disengage when air pressure drops, fail-safe in the event of leaks or failures. That’s why commercial trucks and buses overwhelmingly use air brakes instead of hydraulics.

However, air brakes require more frequent adjustments than hydraulic brakes. The pads tend to wear down faster under heavy use, creating slack that must be manually taken up by turning the slack adjuster. Let’s look at how to inspect and adjust them properly.

Step 1: Park the Truck Safely

First and foremost, you need to park the truck somewhere stable:

  • Find a flat, level, solid surface – avoid soft ground or slopes.
  • Make sure there are no objects or obstructions in front or behind the truck.
  • Completely shut off the engine before exiting the cab.
  • Set the parking brake firmly.
  • Chock the wheels using sturdy wheel chocks to prevent any rolling.

Follow all safety precautions as if you were changing a tire. Set cones or warning signs if parked near active roadways. Wear gloves, eye protection, and steel-toed boots in case you drop any tools.

Step 2: Drain the Air Tanks

With the parking brake engaged, locate the air tank drains under the truck chassis. Most trucks have multiple air tanks to hold compressed air.

Turn the drain valves to release all the stored pressure. The “air down” light should turn on the dash when tanks are empty.

Draining the tanks provides slack so you can easily adjust the brake pads. Make sure tanks are completely empty before proceeding.

Step 3: Inspect the Slack Adjusters and Brake Pads

Slack adjusters are mechanisms that convert the pneumatic pressure into mechanical force applied to the brake pads. Inspect each slack adjuster:

  • Locate the slack adjuster on each brake chamber. There should be one on every axle.
  • Confirm the slack adjuster and brake pads are dry and debris-free. Wipe down if needed.
  • Check that nothing is bent, badly worn or damaged. Replace parts as needed.

While inspecting, also check the brake pads on each brake chamber:

  • Pads should be at least 1/4 inch thick. If thinner, they need replacement.
  • Make sure pads are snugly aligned with brake drums. Misalignment can cause uneven wear.
  • Spray away any oil contamination using brake cleaner.

Replace dangerously worn pads immediately. Otherwise, you can simply adjust their tightness against the drum.

Step 4: Adjust the Slack Adjusters

Here is the key process for tuning up your air brake slack:

  1. Release the adjuster lock nut using a wrench. This allows the adjuster to turn.
  2. Locate the adjustment nut and use a wrench to turn it clockwise. Spin it until brake pads lightly touch the drum.
  3. Tighten just until contacting – do not overtighten! You should be able to spin the wheel by hand when done.
  4. Turn the adjusting nut 1/2 turn counterclockwise to provide correct slack.
  5. Tighten the adjuster lock nut to hold that position.

Make sure you don’t over-tighten, as that can cause brake drag and rapid wear. The right amount of slack leaves pads close to the drum without constant contact.

Repeat the adjustment process on every slack adjuster to tune up the entire air brake system.

Step 5: Test Brake Operation

With adjustments complete, inflate the air tanks and test brake operation:

  • Refill air tanks to governor cut-out pressure (typically 120-140 psi).
  • Release the parking brake and press brake pedal.
  • Perform a tug test on each wheel to check for drag. Wheel should spin freely.
  • Road test the truck, slowly applying brakes to ensure proper operation.
  • Adjust further if brakes are grabbing, dragging, or still imbalanced after testing.

Tuning the air brakes is often an iterative process of adjustment, testing, and re-adjustment until braking feels smooth and even. Don’t worry about getting it perfect in one go.

Pro Tips for Adjusting Air Brakes

Here are some professional tips and best practices to keep in mind:

  • Check slack daily or every 500 miles. As pads wear, slack needs ongoing adjustment. Don’t wait until issues arise!
  • Before adjusting, clean components and lubricate slack adjusters. Dirt and corrosion can prevent proper operation.
  • For rear brakes, push the vehicle forward and backward to center the shoes before adjusting.
  • Normal brake stroke range is 1.5 – 2 inches from slack to full braking.
  • Always road test after adjustments. Emergency brake only if anything feels unsafe!
  • If one wheel is grabbing, the others likely need adjustment too. Do all brakes, not just one.
  • Automatic slack adjusters are becoming more common but require special tools. Leave those to the pros!

With experience, you’ll get a feel for the right tightness and spacing. Maintaining your air brakes prevents wear and failure down the road.

FAQs About Adjusting Air Brakes

Can I adjust air brakes myself or do I need a mechanic?

You can adjust air brakes yourself if you understand the system and have the proper tools. However, for major repairs or full brake jobs, it’s wise to visit a certified mechanic.

How often should I adjust truck air brakes?

Check and adjust slack adjusters at least every 500 miles. Also inspect when replacing pads or servicing other brake components.

My brakes squeal loudly. Does that mean they need adjustment?

Not necessarily! Squealing is typically caused by glazing, oil contamination, or lack of lubrication. Try cleaning and lubricating pads first before adjusting.

What’s the downside of having brakes overly tight?

Over-tightening brakes causes drag, rapid wear, overheating of drums and pads, and potential brake failure. Always allow proper running clearance.

Can I just keep tightening the adjusters when pads get thin?

No. If pads are worn down too far (below 1/4 inch), they must be replaced immediately for safety, before adjusting slack.

What type of wrench do I need for adjusting the slack adjuster?

Typically a 1/2 or 9/16 inch wrench works for most manual slack adjusters. Make sure you have the proper sized wrench before starting.

Should I replace automatic slack adjusters with manual ones?

No need. When maintained properly, both provide the right amount of slack. Automatic types just require special procedures beyond DIY adjustment.

What happens if I drain the air tanks before parking the truck?

Brakes could drag and overheat without the air pressure that releases them. Always set the parking brake before draining tanks for service.


Adjusting air brakes may seem complicated at first, but really just comes down to:

  • Parking safely, draining tanks, and inspecting components
  • Spinning the adjuster nuts to provide the right pad clearance
  • Carefully road testing for smooth, even braking

Following the steps here and practicing on your own truck, you’ll quickly become proficient at tuning up your air brakes. Just remember to focus on safety, work methodically, and don’t over-tighten. Mastering DIY air brake adjustments will save you time and money over the long haul.

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