1. Brake Fundamentals
Chapter 57

2. Objectives
Explain the basic principles of braking, including friction, pressure, and heat dissipation
Describe hydraulic system operation, including master cylinder, control valves, and safety switches
Understand the operation of power brakes

3. Introduction Kinetic energy: energy that wants to stay in motion
Apply brakes to stop a car: dry friction changes motion energy to heat energy
Temperature in brake linings can be 600°F
Friction resists movement between surfaces
Coefficient of friction varies
Temperature, rubbing speed, surface condition
During a stop
Vehicle weight shifts to front brakes
Front breaks wear out faster

4. Brake Linings Linings are bonded or riveted to disc backing
Newer pads integrally molded
Lining types
Asbestos linings: health hazard
Semimetallic linings: sponge iron and steel fibers
Metallic linings: used in heavy-duty and racing conditions
Ceramic linings: use ceramic and copper fibers to control heat

5. Drum and Disc Brakes Drum brake systems Disc brake systems
Metal brake drums bolted to wheels
Disc brake systems
Rotor and caliper, similar to bicycle

6. Hydraulic Brake System Operation
Brake pedal depression
Moves piston in master cylinder
Fluid under pressure is pushed to slave cylinder
Slave cylinders are located at each wheel
Pascal’s Law:
Pressure in an enclosed system is equal and undiminished in all directions
Force = Pressure x Area
Force applied to brake linings increases with larger diameter wheel cylinder

8. Hydraulic Brake Fluid Glycol-based fluids are hygroscopic Brake fluid
Absorb water
Brake fluid
Higher boiling point than water
DOT specifications
List both dry and wet boiling points

9. Brake Hose and Tubing Steel hydraulic brake tubing Brake lines
Runs the length of the vehicle
Rubber hoses connect steel tubing to other components
Flexibility needed to allow wheels to pivot
Brake lines
Made of double-walled steel tubing coated with rust-preventative material
Replacing brake lines: copy originals as closely as possible

11. Hydraulic System Operation
Driver depresses the brake pedal
Linkage applies force to piston at rear of master cylinder
Master cylinder operation
Supplies hydraulic pressure to wheel cylinders
Primary cup compresses fluid when pedal is depressed
Secondary cup keeps fluid from leaking out
Seal lips are directional
Seal installed backwards will leak

12. Low Brake Pedal Low pedal Tandem master cylinder
Brake pedal moves closer to floor before brakes applied
Tandem master cylinder
Cylinder bore with two pistons and chambers
Master cylinder reservoirs
Prevented from vacuum locking
Rubber diaphragm in cover or plastic float
Master cylinders
Mounted on bulkhead

13. Split Hydraulic System
Longitudinally split system
Front and rear brakes: separate hydraulic systems
Used on rear-wheel-drive vehicles
Diagonally split system
Operates brakes on opposite corners of vehicle
Used on front-wheel-drive vehicles
Front suspension geometry
Negates brakes’ tendency to pull to one side

14. Quick Take-Up Master Cylinder
Some disc brake calipers are designed to have less drag when brakes are not applied
More fluid needed to take up clearance
Quick take-up master cylinder
Moves larger amount of fluid when pedal first applied
Rear of primary piston larger diameter than front
Larger part of bore allows piston to move large volume of fluid more quickly

16. Drum Brakes Found in some rear brake applications
Good initial stopping
Inexpensive, mechanical parking brake
Dual-servo drum brake
Self-energizing: during stopping, leading shoe digs into brake drum
Servo action: small force applied to make larger force
Leading-trailing brake
Non-servo brake with anchor at bottom end of each shoe

17. Drum Brake Adjustment
Brakes wear: clearance increases between lining and drum
Typical drum brake adjust has threaded shaft attached to integral starwheel
Dual-servo self-adjusters operate when brakes are applied during a stop when backing up
Brake fade: results with excessive brake heat
Drum brakes do not dissipate heat as well as disc brakes
Increased heat causes drum to expand
More effort required to stop the car

18. Disc Brakes Disc brake system has rotor and caliper
Caliper clamps friction pads against rotor
Rotors are solid or ventilated
Lightweight solid used in lighter cars
Ventilated have more surface area
Used in heavier vehicles
Brake calipers
Fixed caliper: pistons on both sides
Floating caliper: one to two pistons on one side

19. Disc Brakes (cont'd.) Caliper pistons hollow and cup-shaped
Installed with open side against friction pad back
Rear disc brake systems
Have fixed or floating calipers
Linings are fastened to metal back
May have tabs on pad back that need to be bent during installation
Some include wear sensor
Metal tab rubs against rotor when lining wears thin

20. Hydraulic System Valves and Switches
Tandem systems have a hydraulic safety switch
Alerts drivers when half the system fails
Some master cylinders have a fluid level switch
Several designs

21. Hydraulic Control Valves
Metering valve
Used on front disc brakes when car has rear drum brakes
Prevents front brakes applying until rear shoes overcome spring pressure and contact drums
Unnecessary with four-wheel disc brakes
Proportioning valves
Prevent rear wheels from locking during hard stop
Newer cars
Equipped with antilock brakes

22. Power Brakes Brake booster Vacuum-suspended power brake
Allows master cylinder to have larger bore
Brakes apply with less pedal travel
Has check valve to provide reserve braking
Vacuum-suspended power brake
Metal chamber divided by rubber diaphragm
Air enters through filter behind pedal pushrod boot
Other power brake types:
Hydraulic power assist, electric power assist, and hydro-boost systems

23. Parking Brake Must operate independently of service brakes
Cable connected hand brake or foot brake and to an equalizer
Cable from each rear wheel is attached to both sides
Pivots in center and applies each rear parking brake equally
Warning light indicates when brake is applied
Helps prevent damage to braking system

24. Types of Parking Brakes
Drum brakes use integral-type parking brake
Cable-actuated bar applies drum-type brake
Drum-in-hat brake uses miniature drum and shoes housed in rotor center
Parking brake may be integral to rear disc service brakes
Independent-type emergency brake
May be internal-expanding type or external-contracting type

25. Stoplight Switches and Antilock Brakes
Stoplights are turned on by a stoplight switch
Pedal is depressed
Contacts complete circuit
Antilock brake systems (ABS) keep wheels from locking up
Sensors and computer monitor wheel speed
Hybrid vehicle brake systems have same parts as conventional systems
Regenerative braking and computer controls operate hydraulic brake