How to Check Slack Adjusters | A Full Step by Step Guide

How to Check Slack Adjusters

Proper adjustment of the adjusters is essential for the trailer brakes to function properly. Adjustable elements compensate for the normal wear of the brake linings. The clearance adjuster is an arm attached to the pushrod on the brake unit. Clearance adjustment means that the pushrod in the air chamber can be easily adjusted to a predetermined tolerance, maintaining optimal braking performance. Let’s learn how to check slack adjusters.

How to Check Slack Adjusters

How to Check Slack Adjusters

It is important to lock the tires before the inspection so that the trailer cannot move. The best way to prevent movement is to lift the vehicle on the jack stands while adjusting.

Release the brake system at full pressure, release the emergency brake and press the air valve to release the brakes. Listen for air leaks, as this can reduce your braking performance. Mark the push rod’s departure point from the air chamber with a piece of chalk.

Adjust the manual clearance screw. It links the cam and the pushrod to the axle box’s air chamber. Turn the slack adjuster with an appropriate wrench. Follow the s-cam to see which direction it moves to make sure you set it up correctly; once the push rod is pushed out of the air chamber, you are wrongly adjusting it.

Continue to tighten until you feel resistance, which indicates that the brake pads have been drawn near the drum.

Turn the clearance adjuster a quarter to a half turn so that there is a small gap between the drum and the liner.

Pressurize the system and brake with normal pressure. You should not have more than a tenth of an inch between the brake lining and the drum, and the pushrod travel should be no more than 2 inches.

How to Replace the Brake Air Clearance Adjuster?

How to Check Slack Adjusters

Adjust the air brakes and park the truck in a safe location. Place the wedges under both back tries to keep the vehicle from rolling away.

Get back inside the truck and depress the air brake lever to release the air brakes. Locate the air tank on the truck’s side and slowly release the valve to let extra air out of the system. This clears the braking system of all air and frees the moveable adjustments. Maintain the open position of the lever.

Crawl beneath the truck’s bed to locate a turnbuckle. Turn the adjustment screw head on the back of the suspension adjuster counterclockwise to loosen it. The suspension adjustment is now free of the S-cam.

Disconnect the connecting rod between the backlash adjuster and the air brake chamber by removing the pin. Begin by removing the pin from the rod’s end using pliers. Then, using a rubber mallet, tap the connecting rod’s end. 

Remove the circlip on the clearance adjuster side with a flat-blade screwdriver. The cam bar S is held in the play adjuster by this circlip.

Using a rubber mallet, tap the side of the trailer tensioner toward the end of the cam rod S until the tensioner is released. Also, remove the old adjuster on the S-cam rod and replace it with the new one.

To install a new backlash adjuster:

  • Use the same methods as before. 
  • Return to the air tank and shut the handle after the new adjusters are in position. 
  • Wait for the air pressure to restore to at least 120 psi before turning the cart up. 
  • Pull back on the air brake lever.

Return to the truck and paint the connecting rod that connects to the air brake chamber’s rear.

Move to the truck’s front and depress the air brake lever. Turn to the adjusters and locate the connecting rod pin mark. The identification should be no less than two inches away from the compressed air brake chamber’s back.

Pull the air brake lever to re-adjust the air brakes, and then switch off the engine. Eliminate the wheel chocks from the rear of the wheels.

Frequently Asked Questions

How often should the adjusters be greased?

Lubricate the idler pulley with an approved lubricant every six months or 50,000 miles. A grease screw on the adjusters enables lubrication during normal chassis maintenance.

Define a Brake clearance adjuster 

On cam-type brakes, clearance adjusters are mechanical connectors between the chamber pressure rod and the camshaft. When the brakes are applied or released, the full clearance adjuster spins with the brake camshaft as a unit.

Is it okay to set the auto adjusters manually?

Automatic slack adjusters that are properly maintained require no manual adjustments. You will only need to adjust the automatic brake adjuster during a base brake repair or initial installation.

What is the lifespan of the regulators?

The adjusters have a finite lifespan; they do not outlast the vehicle.

Depending on the application, the adjusters should be replaced after four to five years of usage.

When should the auto adjuster be manually set?

The friction surfaces adapt and compensate for the play in the stroke as it wears. A manual adjustment would probably need to be adjusted every 10,000 miles in the “typical” highway environment in the United States.

What is the procedure for inspecting the air brake adjusters?

Completely decelerate and re-mark the pushrod to calculate the push rod’s stroke length and measure between the two markers. If the brake needs to be adjusted, compare the actual stroke to the suggested maximum stroke of 1 1/2 inches (38 mm).

How do you adjust the controls?

Locate the adjustment mechanism on the clearance adjuster. Usually, a 9/16 wrench is needed to turn. Tighten it as far as possible; you should see the Slugs move and pull the brake shoes toward the drum. Then loosen it half a turn, and you should be fine.


Proper control of the slack adjusters on the truck is essential for safe truck operation. Clearance adjusters are meant to continually keep the brakes in the proper position during regular operation. However, they should be checked daily to ensure they continue to perform well.