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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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