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Chapter 71

Chapter 71. Brake System Fundamentals. Automotive Brakes. Provide a means of using friction to either slow, stop, or hold the wheels of a vehicle When a car is moving, it has energy stored in the form of inertia (kinetic energy)

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Chapter 71

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  1. Chapter 71 Brake System Fundamentals

  2. Automotive Brakes • Provide a means of using friction to either slow, stop, or hold the wheels of a vehicle • When a car is moving, it has energy stored in the form of inertia (kinetic energy) • To stop the vehicle, the brakes convert kinetic (moving) energy into heat

  3. NON-Friction Brakes • Engine Braking (Jake Brakes) • Opens exhaust valve during compression stroke • Exhaust brake • Butterfly valve restricts exhaust flow • Magnetic • Electric motor generates electricity • Used on hybrid cars

  4. Drum and Disc Brakes

  5. Typical Braking Ratios • Rear wheel drive vehicles: • front brakes may handle 60% to 70% of the braking, rear brakes handle 30% to 40% • Front wheel drive vehicles: • more weight is concentrated on the front wheels • braking ratio is even higher at the front wheels

  6. Hydraulic System Action Hydraulic jack operation

  7. Hydraulic Brake Action

  8. Dual Master Cylinder

  9. Piston and Cup

  10. Brake Systems Often used on rear-wheel-drive vehicles Often used on front-wheel-drive vehicles, with high front to rear brake ratio

  11. Vacuum Booster Operation

  12. Brake Fluid • Brake fluid must have the following characteristics: • correct viscosity at all temperatures • high boiling point • noncorrosive • water tolerant • lubricates components • low freezing point

  13. DOT 3 • Lowest boiling point • DOT 4 • Middle boiling point • DOT 5 • Highest boiling point • Can mix DOT 3 & 4 • Can not mix DOT 5

  14. Brake Hoses • Hoses • made of reinforced rubber • used where flexing action is necessary

  15. Brake Line • Lines • made of double wall steel tubing • tubing ends use double flare or ISO flare (international standards organization)

  16. Single Piston Caliper Caliper slides as the piston moves in to clamp the brake pads against the rotor

  17. How disc brakes work

  18. Caliper Operation Brakes applied Brakes released

  19. Disc Brake Pads • Brake pad riveted or bonded to backing plate • Linings • organic • semi-metallic • Ceramic

  20. Drum Brake Assembly

  21. Brake Shoe Adjusters Cable-type star wheel adjuster with an overtravel spring

  22. Brake Shoe Adjusters Lever-latch adjuster

  23. Stop Light Switch • Operates the rear brake lights • Normally open switch, usually mounted on the brake pedal mechanism • When the brake pedal is pressed, it closes the switch

  24. Parking Brake Components Foot-operated parking brake pedal Lever pushes the shoes against the drum

  25. Rear Disc Brake Caliper Note the parking brake mechanism

  26. Chapter 72 Brake System Diagnosis and Repair

  27. Brake Vibration • Shows up as a chatter, pulsation, or shake in the brake pedal or steering wheel • Occurs only when the brakes are applied • Front brake problems may be felt mostly in the steering wheel • Pulsation in the brake pedal may be caused by brake problems at any wheel

  28. Brake Vibration • Possible causes: • warped disc • out-of-round brake drum • hard spots on the disc or drum

  29. Grabbing Brakes • Brakes apply too quickly, even with light brake pedal application • Possible causes: • malfunctioning brake booster • brake fluid or grease on the linings • worn brake linings • faulty metering valve • mechanical problem in the wheel brake assembly

  30. Excessive Pedal Effort • Noticeable increase in the amount of foot pressure needed to apply the brakes • Possible causes: • seized wheel cylinder or caliper piston • collapsed brake hose or crimped line • faulty master cylinder • contaminated linings • disconnected brake booster vacuum line or defective booster

  31. Pulling Brakes • Vehicle veers to the right or left when the brakes are applied • Possible causes: • seized caliper or wheel cylinder piston • grease or fluid-coated lining • leaking cylinder • faulty automatic adjuster • brake lining dust in a drum brake assembly

  32. Spongy Brake Pedal • Brakes will apply, but the pedal does not feel solid • Possible causes: • air in the brake system • faulty residual check valve in the master cylinder • maladjusted brake shoes

  33. Dropping Brake Pedal • Pedal slowly moves all the way to the floor when steady pressure is applied to it • Pumping usually restores pedal height momentarily • Possible causes: • internal leak in the master cylinder • external fluid leak in any component

  34. Low Brake Pedal • Pedal travels too far toward the floor before braking • Possible causes: • inoperative brake adjusters • maladjusted master cylinder push rod • mechanical problem in the wheel brake assemblies

  35. Dragging Brakes • Brakes remain partially applied when the brake pedal is released • To detect dragging brakes, carefully feel each brake assembly • dragging brakes will be abnormally hot

  36. Dragging Brakes • Possible causes: • seized wheel cylinder pistons or parking brake cables • overadjusted parking brake or master cylinder push rod • weak return springs • master cylinder problems

  37. No Brake Pedal • Very dangerous condition in which the brake pedal moves to the floor with no braking action • Possible causes: • hydraulic system leak which causes the loss of the brake fluid • faulty master cylinder • Complete loss of braking is unlikely on a system with a dual master cylinder

  38. Brake WarningLight On • Indicates either an internal leak (master cylinder) or an external leak • Unequal pressure in the dual master cylinder system has caused the warning light switch to shift to one side

  39. Braking Noise • Grinding Noise • Metal-on-metal sound when braking • caused by worn brake linings

  40. Braking Noise • Scrapping Noise • Light scrapping noise, goes away when brakes are applied • Caused by wear indicator rubbing on rotor

  41. Disc Brake Inspection Inspect brake pad linings (minimum thickness 1/8” or 3 mm)

  42. Disc Brake Inspection Inspect rotors for scoring or heat damage

  43. Drum Brake Inspection Check the drum for cracks or heat damage A badly scored drum must be machined

  44. Braking Noise • Squeak when braking • caused by glazed or hardened brake linings, dry brake drum backing plate, or wear indicator rubbing on the rotor • Rattles • caused by missing anti-rattle clip or loose parts

  45. Brake System Inspection • Most manufacturers recommend a periodic inspection of the brake system • When inspecting the brake system, check the brake pedal action, fluid level, and the condition of the brake lines, hoses, and wheel brake assemblies

  46. Checking Brake Fluid

  47. Checking for Leaks • Inspect for leaks whenever the fluid level in the master cylinder is low • Check brake lines, hoses, calipers, and wheel cylinders • Check below the master cylinder mounting point at the cowl or booster • Leaks show up as darkened, damp areas

  48. Checking the Parking Brake • Apply the parking brake • Pedal or lever should not move more than 2/3 of full travel • Parking brake should keep the vehicle from moving with the engine idling and the transmission in drive • Inspect cables for fraying and high friction

  49. Master Cylinder Service • A faulty master cylinder may leak fluid past the rear piston or leak internally • Inspect for external leakage where the master cylinder mounts to the booster or cowl • Internal leakage will cause the pedal to slowly sink to the floor when applied

  50. Measuring Disc Thickness Using an outside micrometer, measure at several points around the disc

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