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Problem: Improper rear drum brake adjustment.

Cause: Not understanding when and how self-adjustment takes place on duo-servo and non-servo drum brake equipped vehicles.

Solution: Learning to identify the self-adjusting parts and when they operate will allow the following:

  • Determining whether the self-adjusting mechanism is operating as designed.
  • Allow proper assembly or repair of self-adjustment mechanism
  • Identify when self-adjustment takes place

Duo-Servo Drum Brakes 

Duo-servo drum brakes can be identified by the location of either the self-adjuster or anchor pin. The self-adjuster is located at the bottom between the primary and secondary brake shoe. Likewise, the anchor pin is located at the top between the two brake shoes.

All duo-servo drum brakes are designed to self-adjust when backing up only if the following conditions are met:

  • If there is a large enough gap between the secondary brake shoe and brake drum.
  • if the parts that make up the self-adjusting mechanism are operating properly.

When servicing vehicles with duo-servo drum brakes it is important to make sure all parts are lubricated and installed correctly.

Non-Servo Drum Brakes 

The majority of FWD vehicles are equipped with non-servo drum brakes. The most distinguishing feature of a non-servo drum brake is the location of the anchor pin. The anchor pin on non-servo drum brakes is located at the bottom of the backing plate.
There are two techniques used to allow self-adjustment on non-servo drum brakes. The most common is during forward braking. The vast majority of FWD vehicles use this method. There are a small number of non-servo drum brakes that use the parking brake to self-adjust.
A good rule of thumb to follow is to use the location of the self-adjuster to help in determining when self-adjustment takes place. If the self-adjuster is incorporated into the strut rod as shown on Figures 60.1 & 60.2 self-adjustment takes place during forward braking. If the self-adjuster is incorporated into the parking brake lever as shown in Figure 60.3 then self-adjustment takes place during parking brake use.
While these rules of thumb are pretty accurate they do not apply to all cases. Another way to determine when self-adjustment takes places is to engage both methods and see which one works.

Screen Shot 2016-05-24 at 1.25.57 PM Figure 60.1 Screen Shot 2016-05-24 at 1.26.14 PM Figure 60.2

Screen Shot 2016-05-24 at 1.26.26 PM Figure 60.3

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