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Chapter 6

Chapter 6. Drum Brake Fundamentals and Service. Introduction. Until the 1960s all vehicles used drum brakes on all four wheels. Many vehicles have disc front and drum rear, or hybrid brakes. Increasingly, new vehicles use disc brakes on all four wheels. Drum Brake Parts. Brake Linings.

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Chapter 6

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  1. Chapter 6 Drum Brake Fundamentals and Service

  2. Introduction • Until the 1960s all vehicles used drum brakes on all four wheels. • Many vehicles have disc front and drum rear, or hybrid brakes. • Increasingly, new vehicles use disc brakes on all four wheels.

  3. Drum Brake Parts

  4. Brake Linings • Brake linings are either: • Bonded to the brake shoe. • Riveted to the brake shoe. • Riveted linings are generally better quality.

  5. Drum Brake Lining Terms

  6. Brake Lining Terms • The primary lining can be shorter than the secondary — if so it must be installed to the front side for proper brake operation.

  7. Wheel Cylinder Parts

  8. Drum Brake Designs There are two main types of drum brakes: • Duo-servo. • Leading-trailing.

  9. Duo-Servo Duo-servo brakes are self-energizing. • The leading (primary) shoe is forced into the rotating drum by the wheel cylinder. • The primary shoe wedges the secondary shoe against the drum. • The secondary shoe is also shoved against the anchor at the top.

  10. Leading-Trailing Brakes • Only the leading shoe is self-energized. • Wheel cylinder action wedges leading shoe into the brake drum, but anchor at bottom prevents trailing shoe from servo action. • See Figures 6.14 to 6.16 for both styles of brakes.

  11. Drum Brake Adjustment • Duo-servo brakes self-adjust when stopping in reverse. • Leading-trailing brakes self-adjust any time there is excessive clearance. • Drum brakes may have to be manually adjusted periodically.

  12. Drum Brake Service • Inspect for signs of grease or brake fluid leaks. • Look for unusual wear conditions. • Use a safe method to clean brakes before disassembly.

  13. Rebuilding Wheel Cylinders • It is possible to rebuild wheel cylinders, but it is usually more cost-effective to replace. • Aluminum wheel cylinders must not be honed.

  14. Inspecting and Cleaning Brake Parts • Clean and carefully inspect all parts. • Replace any part that is questionable. • Make sure springs are not stretched. • Any part replaced should also be replaced on the opposite side — for equal braking.

  15. Ordering Replacement Parts Before calling for parts have these ready: • Vehicle Identification Number (VIN). • Vehicle production date.

  16. Replacement Parts • Be sure to use quality parts — vehicle brakes are NOT the place to save money!

  17. Brake Reassembly • Carefully inspect your work. • Some techs do only one side at a time to check their installation of parts. • Set initial lining clearance. • Test drive the vehicle to verify safe repair.

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