This chapter discusses drum brakes, including their components, operation of non-servo and dual-servo drum brakes, advantages and disadvantages of drum brakes, and automatic brake adjusters. Drum brakes use shoes that press against the inside of a drum to create friction and slow the vehicle. Non-servo brakes apply each shoe individually while dual-servo brakes use one shoe to help apply the other, increasing braking force. Automatic adjusters compensate for brake pad wear to maintain proper clearance between pads and drum.

1. CHAPTER 10 Drum Brakes

5. DRUM BRAKE ADVANTAGES FIGURE 10–1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.

6. DRUM BRAKE ADVANTAGES FIGURE 10–2 An exploded view of a typical drum brake assembly.

13. DRUM BRAKE PARTS BACKING PLATE FIGURE 10–4 A labyrinth seal is created between the lip of the backing plate and the groove in the brake drum.

18. DRUM BRAKE PARTS WHEEL CYLINDERS FIGURE 10–8 The pushrods are held in place by the rubber dust boots. As the wheel cylinder pistons move outward, the pushrods transfer the movement to the brake shoes.

20. DRUM BRAKE SHOES FIGURE 10–11 Typical drum brake shoe and the names of the parts. (Courtesy of Allied Signal Automotive Aftermarket) FIGURE 10–10 Tapered ends on the linings help to reduce brake noise.

22. DRUM BRAKE SHOES PRIMARY AND SECONDARY BRAKE SHOES FIGURE 10–13 Primary shoe lining may vary depending on the application. FIGURE 10–12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing).

24. DRUM BRAKE SHOES LINING ASSEMBLY METHODS FIGURE 10–15 Many brake linings are bonded.

26. DRUM BRAKE SHOES EDGE CODES

27. DRUM BRAKE SHOES EDGE CODES FIGURE 10–16 Typical drum brake lining edge codes.

30. DRUM BRAKE PARTS RETURN SPRINGS FIGURE 10–18 A single spring-steel spring is used on some drum brakes.

32. DRUM BRAKE PARTS BRAKE SHOE HOLDDOWNS FIGURE 10–19 Various types and styles of hold-down springs.

37. NON-SERVO BRAKE DESIGN PARTS AND OPERATION FIGURE 10–22 Self-energizing action can increase or decrease the stopping power of a brake shoe.

42. DUAL-SERVO BRAKE DESIGN DUAL-SERVO BRAKE CONSTRUCTION FIGURE 10–26 A typical dual-servo brake adjusting link assembly.

45. DUAL-SERVO BRAKE DESIGN SERVO ACTION FIGURE 10–28 Dual servo action greatly increases the application force on the secondary shoe.

48. AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–29 A cable-actuated starwheel adjuster. This type of adjuster makes the adjustment as the brakes are released.

49. AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–30 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment when the brakes are applied.

50. AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–31 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.

51. AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–32 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.

52. AUTOMATIC BRAKE ADJUSTERS SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–33 A cable-actuated starwheel adjuster with an over-travel spring.

54. AUTOMATIC BRAKE ADJUSTERS NON-SERVO STARWHEEL AUTOMATIC ADJUSTERS FIGURE 10–35 A non-servo brake with a lever-actuated starwheel automatic adjuster on the trailing shoe. This type of adjuster makes the adjustment as the brakes are released.

57. AUTOMATIC BRAKE ADJUSTERS RATCHET-TYPE AUTOMATIC ADJUSTERS FIGURE 10–36 A lever-latch ratchet automatic adjuster.

58. AUTOMATIC BRAKE ADJUSTERS RATCHET-TYPE AUTOMATIC ADJUSTERS FIGURE 10–37 A strut-quadrant ratchet automatic adjuster.