Brake & Wheel End

What you need to know about slack adjusters, so your braking system can perform at its best.

Everything You Need To Know About Slack Adjusters

What is a slack adjuster and why is it important to your vehicle’s braking system? We get this question a lot at Wheelco. The slack adjuster is the component that is used to adjust the brakes using transfer of torque by converting convert linear motion into the rotary motion of the S-cam.

How Do Slack Adjusters Work?

The slack adjuster assembly transmits the braking torque to the S-cam and facilitates the adjustment of clearance between the brake lining (brake pad) and the brake drum.  It is mounted on the S-cam shaft. One end is connected to the brake chambers’ pushrod. So, when the driver presses down on the brake pedal, air is forced into the brake chamber and shoves the push rod out. Once this happens, it rotates the slack adjuster which forces the brake shoes out. The brake shoes press against the brake drum, applying the brake. 

Slack Adjuster Preventative Maintenance and Inspection

When the brake lining or brake pad wears, the gap between it and the brake drum increases. Automatic slack adjusters have an adjusting mechanism that adjusts for brake lining wear. However, manual slack adjuster periodically needs to be adjusted to properly function and operate at peak performance and compensate for lining wear. At Wheelco, we suggest adjusting your manual slack adjuster approximately every 10,000 miles.  However, this is just a general guideline since truck braking systems vary. When in doubt, call our service technicians for further assistance.

Manual Slack Adjusters

After 1994, all vehicles made have automatic slack adjusters installed. However, if you are driving a vehicle that predates this regulation, manual adjusters require fine-tuning. Read more about Brake FMSCA Regulations.

Manual Slack Adjuster Periodical Inspection Procedure:

  • Park the truck or trailer on level ground
  • Ensure the parking brake if OFF so can move the slack adjusters
  • Pull on the slack adjuster. If it moves more than 1 inch or 25mm where the pushrod is attached it most likely requires adjustments
  • Out-of-adjustment brakes is one of the most common problems found in roadside inspection.

Be safe and check the slack adjuster movement. Often, improper adjustment is due to operator negligence. Should you require any additional information, contact Wheelco customer service to assist you.

Automatic Slack Adjusters

Automatic Slack Adjusters operate in the same manner that manual slack adjuster does but with one key difference. They automatically adjust for the gradual brake lining wear. Therefore, it does not require periodic manual adjustment and is highly recommended not to adjust unless absolutely necessary or in an emergency situation.

Facts about automatic slack adjusters:

  • Do not manually adjust automatic slack adjusters after initial adjustment unless you are doing a complete brake overhaul.
  • Never use an impact wrench to adjust. If you do, internal slack adjuster damage may occur.
  • Grease the slack adjuster if required.
  • When re-lining brakes, remove the slack adjuster from the S-cam. Then rotate the worm wheel 90 degrees before refitting. This reduces uneven wear.
  • Ensure the adjustment arm is firmly fitted to the bracket.

Automatic slack adjusters are fast becoming a more reliable and consistent operational part, which is why Wheelco recommends using this type of slack adjusters. There are two types of slack adjusters: the stroke and clearance adjuster.

Stroke Adjuster:                   

The stroke adjuster measures the stroke of the pushrod. It keeps the stroke constant, no matter if you brake hard or soft.

Clearance Adjuster:

The clearance adjuster measures the clearance between the drum and brake lining and keeps the wear consistent (insufficient or excessive clearance between the brake lining and the drum). Once the lining meets the drum the adjusting mechanism disengages. This means that the brake adjusters’ automatic adjustment is not affected by the driver’s braking method (hard or soft).

Wheelco’s Slack Adjuster Recommendations 

Depending on your specific needs we offer several braking solutions.

Meritor Special Features:

  • Low-maintenance, long-life design with fewer moving parts
  • No Slip Clutches to wear out
  • Q Plus LX500 extended-lube feature combines specially mated brake and Meritor ASA

Meritor Part Numbers: R803195, R803110, R803112, R803116

Stemco Special Features:
  • Auto-Check brake adjustment check
  • Cam Guard lubrication system
  • Installation template for error– free Install

Stemco Part Numbers: MK41100S, MK42100S, MK42106S, MK43106S

Gunite Special Features:
  • Maintains consistent brake adjustment
  • Accurately reads the shoe-to-drum clearance
  • Clearance Sensing Adjuster
  • O-Ring on spline gear

Gunite Part Numbers: AS1132, AS1140, AS1141, AS1144 

Why Buy from Wheelco Truck and Trailer?

Our slack adjuster knowledge is immeasurable. We understand that no two customers are alike, nor are their needs. We offer premium products and popular brands. Tell us what you are driving, and we’ll recommend the best slack adjuster for you.