46 Brake Systems Chapter 46

Objectives Explain the basic principles of braking, including kinetic and static friction, friction materials, application pressure, and heat dissipation. Describe the components of a hydraulic brake system and their operation. continued… Chapter 46

Objectives Perform both manual and pressure bleeding of the hydraulic system. Briefly describe the operation of drum and disc brakes. Inspect and service hydraulic system components. continued… Chapter 46

Objectives Describe the operation and components of both vacuum-assist and hydraulic-assist braking units. continued… Chapter 46

Friction Kinetic friction Static friction Kinetic or moving friction is created by brake pads or shoes rubbing against rotating brake rotors or drums. Changes kinetic energy into thermal energy (heat). Static friction Static or stationary friction holds the car in place when stopped and should be present between the tire and road at all times. continued… Chapter 46

Friction continued… Chapter 46

Factors Governing Braking Pressure The amount of friction generated between moving surfaces contacting one another depends in part on the pressure exerted on the surfaces. Coefficient of friction The amount of friction between 2 surfaces (pads and rotors or shoes and drums) Determined by dividing the force required to pull an object across a surface by the weight of the object continued… Chapter 46

Factors Governing Braking Frictional Contact Surface The amount of surface, or area, that is in contact. Simply put, bigger brakes stop a car more quickly than smaller brakes used on the same car. Heat Dissipation The tremendous heat created by the rubbing brake surfaces must be conducted away from the pad and rotor (or shoe and drum) and be absorbed by the air. continued… Chapter 46

Friction Material Classifications Fully metallic Lining is made of sintered iron and has been used for years in heavy-duty and racing applications because they have great fade resistance. They require very high pedal pressure and tend to quickly wear out drums and rotors. continued… Chapter 46

Friction Material Classifications Semi-metallic Made of iron fibres moulded with an adhesive matrix. Offers excellent fade resistance with good frictional characteristics so only a moderate amount of application pressure is needed. Semimetallic pads and shoes do not cause excessive wear on rotor or drum surfaces. continued… Chapter 46

Friction Material Classifications Organic (non-metallic) For many decades, asbestos was the standard brake lining material. It offers good friction qualities, long wear, and low noise. New materials, such as composite/organic, ceramics, and carbon fibres, are being used because of the health hazards of breathing asbestos dust. continued… Chapter 46

Friction Material Classifications Synthetic Non-asbestos lining materials made of synthetic substances are now available. The major brake lining manufacturers are constantly experimenting with new materials that meet all established criteria for long life, friction characteristics, drum and rotor wear, and heat dissipation. continued… Chapter 46

Brake System Components Brake fluid Master cylinder Power booster Control valves Wheel brake assemblies Parking brake ABS system continued… Chapter 46

Hydraulic Principles An example of how hydraulics can increase output force continued… Chapter 46

Dual Hydraulic Brake Circuits continued… Chapter 46

Dual Hydraulic Brake Circuits continued… Chapter 46

How the Master Cylinder Works A pushrod connects the brake pedal to the master cylinder piston. When the pedal is pressed, the piston is pushed forward and the fluid in front of the piston is displaced into the brake system moving the pads and shoes into contact with the drums and rotors. As more force is placed on the brake pedal, the fluid transmits force throughout the system. continued… Chapter 46

Master Cylinder Operation At rest Fluid in the reservoir can flow into the area ahead of the piston through the compensating port. This keeps the system full and prevents pressure build-up during temperature changes. continued… Chapter 46

Master Cylinder Operation Applied The piston is moved forward blocking the compensating port forming a sealed pump chamber. continued… Chapter 46

Master Cylinder Operation Brake release When the pedal is released the piston return springs force the piston back. The piston cup seal collapses and fluid flows past to allow speedy piston return and pressure decrease. continued… Chapter 46

Master Cylinder Components continued… Chapter 46

Lines and Hoses continued… Chapter 46

Brake System Valves Metering valve Proportioning valve Delays application of front disc brakes until pressure builds to rear drum brakes. Proportioning valve Reduces the hydraulic pressure to the rear drum brakes to balance braking. Pressure differential valve Used to operate a warning light switch. Combination valve Combines several functions in one valve. continued… Chapter 46

A Proportioning Valve continued… Chapter 46

Pressure Differential Valve continued… Chapter 46

Drum Brakes A drum brake assembly consists of a cast-iron drum, which is bolted to and rotates with the vehicle’s wheel, and a fixed backing plate to which the shoes, wheel cylinders, automatic adjusters, and linkages are attached. The shoes are surfaced with frictional linings, which contact the inside of the drum when the brakes are applied. The shoes are forced outward by pistons located inside the wheel cylinder. continued… Chapter 46

Drum Brakes continued… Chapter 46

Disc Brakes Disc brakes use a cast iron rotor, inboard of the vehicle wheel. Both sides of the rotor are machined smooth for the brake pads to rub against. Usually the 2 surfaces are separated by a finned centre section for better cooling (ventilated rotors). The pads are attached to metal shoes, which are actuated by pistons housed in the brake caliper. continued… Chapter 46

Disc Brakes continued… Chapter 46

Brake Fluid Inspection Remove the cover and check the gasket. Check the fluid in the reservoir. Do not overfill. Check the fluid for contamination. Check the master cylinder for signs of leaks. continued… Chapter 46

Vacuum Brake Boosters All modern vacuum-assist units are vacuum suspended systems. The diaphragm inside the unit is balanced using engine vacuum until the brake pedal is depressed. Applying the brake allows atmospheric pressure to unbalance the diaphragm and allows it to move generating application pressure. continued… Chapter 46

Vacuum Brake Boosters continued… Chapter 46

Vacuum Booster Checks Pressure check Pedal travel Vacuum reading Release problems Hard pedal Grabbing brakes Internal binding continued… Chapter 46

Hydraulic Brake Boosters Decreases in engine size, increased use of diesel engines, plus the continued use of engine vacuum to operate other engine systems, such as emission control devices, led to the development of hydraulic-assist power brakes. These systems use fluid pressure, not vacuum pressure, to help apply the brakes. continued… Chapter 46

Hydraulic Brake Boosters continued… Chapter 46

Summary The four factors that determine a vehicle’s braking power are pressure, coefficient of friction, frictional contact surface, and heat dissipation. Three types of brake lining materials are used: metallic, semi-metallic, and non-asbestos substances. continued… Chapter 46

Summary Two common brake system control valves are the metering valve and the proportioning valve. The brake lines transmit brake fluid pressure from the master cylinder to the wheel cylinders and calipers. continued… Chapter 46

Summary Common brake problems include hard pedal, low pedal, leaks, grabbing, and longer than normal stopping distance. Brake system service includes checking the fluid, brake lines, and brake pedal movement. Chapter 46