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Raybestos® Training Update

Types of Drum Brake Systems

Drum brake systems are typically categorized as either a duo-servo or non-servo type setup. In this training video, we discuss the difference between these setups and how they operate. 

Duo-Servo Systems: 

Duo-servo brake systems are also known as primary/secondary drum brake systems. This is because there is a primary shoe and a secondary shoe. The primary shoe has less friction material than the secondary shoe. The primary shoe should be installed in the front, and the secondary shoe in the back. 

In duo-servo systems, the shoes link at the top at an anchoring pin and at the bottom at the self-adjuster. When you press the brake pedal on a duo-servo system, the hydraulic pressure from the wheel cylinder pushes the upper parts of the shoes outward. The primary shoe makes contact with the drum first, and the rotation of the drum tries to pull the shoe downward. 

This energy causes the primary shoe to push against the adjuster. The adjuster then forces the secondary shoe outward against the drum. The combination of this outward movement and the rotation of the drum causes the secondary shoe up against the anchoring pin. This cause the shoes to bind against the drum and stops it from rotating. Therefore, the secondary shoe provides the majority of braking power. 

Non-Servo Brake Systems:

The difference between duo-servo and non-servo systems is that the anchoring pin is on the bottom. This means that the application of one shoe has no effect on the other. In non-servo systems, there is a leading shoe and a trailing shoe. The leading shoe is the shoe that moves in the direction the drum is moving. Thus, the leading shoe changes depending on whether you are in forward or reverse.

In either case, the leading shoe provides the majority of the braking force. The rotation of the drum causes the leading shoe to apply with greater force than the trailing shoe.

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