This document discusses the key components of drum brake systems, including wheel cylinders, brake shoes, brake linings, servo action, brake self-adjusters, and drums. It explains that wheel cylinders contain pistons that apply force to the brake shoes, while return springs hold the shoes away from the drum. Brake linings are made of heat-resistant materials and can vary in thickness and shape between shoes. Some systems use servo action to increase the braking force applied. Brake self-adjusters automatically adjust the shoe spacing as the brakes are used. Drums cover the brake mechanisms and provide a surface for shoe contact during braking.

1. Chapter Nine
Drum Brake Components
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2. Drum Brake Components and
Operation
 Wheel Cylinders
 Two pistons, a spring, sealing cups or seals, two links,
and the cylinder body
 Bleeder must be positioned at highest point when
mounting cylinder
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3. Drum Brake Components and
Operation
 Brake Shoes
 Provide mounting area for friction material
 Return springs hold brake shoes away from drum
until force is applied
 Shoes can require mounting instructions
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4. Drum Brake Components and
Operation
 Brake Linings
 Thickness, positioning, and shape may be different on
each lining for one wheel
 Secondary shoe may be shorter and thinner than
primary shoe
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5. Drum Brake Components and
Operation
 Brake Linings
 Brake linings are heat-resistant
 Copper, brass, zinc, and other materials have replaced
asbestos
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6. Drum Brake Components and
Operation
 Servo Action
 Some systems use a stationary anchor to hold lower
end of shoe in place
 Servo systems use force of one shoe to increase force
applied to another shoe
 Decrease stopping distance and force required
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7. Drum Brakes(Leading Trailing)
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8. Drum Brakes(Duo Servo)
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9. Drum Brake Components and
Operation
 Brake Self-Adjusters
 Drum brakes require mechanical adjusters
 Some brake systems require periodic adjustment
 Others adjust shoes automatically as brakes are used
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10. Drum Brake Components and
Operation
 Brake Self-Adjusters
 Automatic adjusters can be actuated by application of
brake pedal or parking brake
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11. Drum Brake Components and
Operation
 Drums
 Cover brake mechanisms
 Provide a location for shoes to press against for
braking
 Interior surface must be machined smooth
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12. Parking Brake Operation
 Holds vehicle in place once it is stopped
 Foot- or hand-operated brakes are used to set the
brake
 Systems have cable attached to hand or pedal
assembly to actuate rear brakes
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