Modern Automotive Technology PowerPoint for by Russell Krick Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois

Chapter 72 Brake System Diagnosis and Repair

Contents Brake system problem diagnosis Brake system inspection Vacuum booster service Hydraulic booster service Master cylinder service Brake system bleeding (Continued)

Contents Brake line and hose service Disc brake service Brake disc (rotor) service Drum brake service Parking brake adjustment

Brake System Problem Diagnosis Gather information from the customer or service writer about any symptoms: smells, abnormal brake pedal movements, or improper braking action Road test the vehicle if necessary Perform a brake inspection Decide what adjustments or repairs are needed

On-Board Diagnostics Self-diagnostics may be available on vehicles, especially those equipped with anti-lock brake systems Connect a scan tool to the diagnostic connector Read any stored diagnostic trouble codes Check the scan data for anti-lock brake operating values

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

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

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

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

Pulling Brakes The 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

Spongy Brake Pedal The 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

Dropping Brake Pedal The 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

Low Brake Pedal The 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

Dragging Brakes The 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

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

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

Brake Warning Light 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

Braking Noise Noises include grinding sounds, squeaks, or rattles Metal-on-metal grinding sound when braking caused by worn brake linings

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

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

Brake System Problems

Checking Brake Pedal Action Apply the brake pedal and compare the movement to specifications Brake pedal application specs: pedal height pedal free play pedal reserve distance

Brake Pedal Measurements

Brake Pedal Measurements Using a yardstick to measure the distance from the steering wheel to the brake pedal

Checking Brake Fluid Remove the master cylinder cover or view the level through the side of the plastic reservoir on some models Typically, the brake fluid level should be 1/4" (6 mm) from the top of the reservoir

Checking Brake Fluid

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

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

Checking Wheel Brake Assemblies Raise the car on a lift or jack stands Remove one front wheel and one rear wheel Inspect the disc brake assemblies Inspect the drum brake assemblies

Inspect the brake pad linings (minimum thickness 1/8" or 3 mm) Disc Brake Inspection Inspect the brake pad linings (minimum thickness 1/8" or 3 mm)

Inspect the rotors for scoring or heat damage Disc Brake Inspection Inspect the rotors for scoring or heat damage

Disc Brake Inspection

Drum Brake Inspection Remove the brake drums The brake shoe linings should be at least 1/16" (1.5 mm) thick Check the linings for glazing or contamination from brake fluid or differential oil

Drum Brake Inspection Check the drum for cracks or heat damage A badly scored drum must be machined Check for leaks, wear, and damaged parts

Vacuum Booster Service Inspect the brake booster and the vacuum hose hoses may collapse or harden To test the booster, pump the brake pedal to remove any vacuum Apply the brake and start the engine the pedal should drop slightly as engine manifold vacuum develops

Removing the Booster

Vacuum Brake Booster

Rebuilding the Booster Following the service manual procedures, replace the diaphragm, the valves, and other plastic or rubber parts Most boosters are rebuilt by specialty shops

Hydraulic Booster Service Check all the hydraulic lines for signs of leakage If the booster is inoperative, check the power steering fluid level Most boosters are not serviceable without special equipment and procedures

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 pressure is applied

Master Cylinder Removal Disconnect the brake lines using a line wrench Unbolt the master cylinder from the booster or cowl On a vehicle without a power-assist booster, disconnect the push rod from the brake pedal assembly

Master Cylinder Rebuild Following the service manual procedures, drain the fluid and disassemble the unit Honing may be used to remove minor corrosion or pits from the cylinder surface must be in perfect condition Measure piston-to-cylinder clearance Inspect each component and replace all cups and seals

Master Cylinder Rebuild Clean and lubricate the components with brake fluid and reassemble the unit

Removes air from inside the master cylinder Bench Bleeding Removes air from inside the master cylinder

Installing a Master Cylinder Bolt the master cylinder to the firewall or booster Check the adjustment of the push rod if specified Install the brake lines with the fittings lightly tightened

Installing a Master Cylinder Bleed the air from the fittings Tighten the fittings Fill the reservoir with fluid Check brake pedal feel and road test the vehicle

Brake System Bleeding Air in the brake system will compress, causing a spongy brake pedal Bleeding involves the use of fluid pressure to force air out of the brake system bleeder screws Two methods are used: manual bleeding pressure bleeding

Manual Bleeding Attach one end of a hose to a bleeder screw and place the other end in a jar containing brake fluid Have another technician apply light foot pressure on the brake pedal Open the bleeder screw while watching for air bubbles flowing from the hose

Manual Bleeding Close the bleeder screw Have your helper release the brake pedal Repeat the steps until the flow from the hose is clear and free of bubbles Perform the operation on all the brake assemblies, starting farthest from the master cylinder

Manual Bleeding

Pressure Bleeding Uses a pressure bleeder tank, which contains brake fluid under pressure

Pressure Bleeding Fill the tank with fluid Charge the tank with 10–15 psi (69–103 kPa) of air pressure Fill the master cylinder with fluid Install the adapter and hose on the master cylinder Open the valve in the hose

Pressure Bleeding Attach one end of a hose to a bleeder screw and place the other end in a jar containing brake fluid Open the screw until the fluid entering the jar is clear of bubbles

Repeat the steps on the other wheel brake assemblies Pressure Bleeding Repeat the steps on the other wheel brake assemblies

Flushing a Brake System Extends the life of the brake system components Done by pressure bleeding all the old fluid out of the system Removes contamination, such as dirt, rust, corrosion, oil, or moisture Bleed each wheel brake assembly until clean fluid flows from the bleeder screw

Brake Line and Hose Service Lines are subject to corrosion or damage Hoses may deteriorate over time When replacing brake lines, use approved double-wall steel tubing

Tools and Procedures

Disc Brake Service Main steps in disc brake service: replacing worn brake pads rebuilding or replacing caliper assemblies turning (machining) the brake discs bleeding the system

Replacing Brake Pads Loosen the wheel lug nuts Raise the vehicle on a lift or jack stands Remove the wheels

Use a large C-clamp to push each piston back into its cylinder Replacing Brake Pads Use a large C-clamp to push each piston back into its cylinder

Unbolt the calipers and slide them off the discs Replacing Brake Pads Unbolt the calipers and slide them off the discs

Replacing Brake Pads Support the calipers to prevent hose damage, and then remove the old pads

Replacing Brake Pads Install the anti-rattle clips on the new pads and fit them into the calipers

Replacing Brake Pads Slide the caliper assemblies over the discs Assemble the mounting hardware Make sure all bolts are torqued to specs Install the wheel and tighten the lug nuts to specs

To remove the piston, apply low air pressure Rebuilding a Caliper To remove the piston, apply low air pressure

Pry the old dust boot out of the caliper Rebuilding a Caliper Pry the old dust boot out of the caliper

Remove the old seal from the caliper Rebuilding a Caliper Remove the old seal from the caliper

Keep the parts organized Rebuilding a Caliper Keep the parts organized

Rebuilding a Caliper Honing can remove minor imperfections Lubricate the hone with brake fluid

Rebuilding a Caliper Inspect the piston for wear or damage Clean all parts in an approved cleaner Coat the parts in brake fluid

Work the new seal into the bore groove Rebuilding a Caliper Work the new seal into the bore groove

Rebuilding a Caliper Install the new boot in its groove and slide the piston squarely into the caliper

Brake Disc (Rotor) Service It is important to check the condition of the disc Manufacturers provide specifications for maximum disc runout and minimum disc thickness

Measuring Disc Runout Runout is the amount of side-to-side movement measured near the outer edge of the friction surface Typical maximum runout: 0.004" (0.10 mm) If runout is beyond specs, turn the disc on a brake lathe to true its friction surfaces

Using a dial indicator, rotate the disc while watching needle movement Measuring Disc Runout Using a dial indicator, rotate the disc while watching needle movement

Measuring Disc Thickness The thickness will decrease as the disc wears The minimum thickness may be printed on the side of the disc If the disc thickness is under specifications, replace the disc

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

Resurfacing a Disc Involves machining a disc’s friction surfaces on a brake lathe to correct runout, thickness variation, or scoring Mount the disc on the brake lathe according to the lathe manufacturer’s recommendations With the machine feeds and controls set properly, machine smooth surfaces on the disc

Always read the operating manual before using a brake lathe Resurfacing a Disc Always read the operating manual before using a brake lathe

Typical rotor mounting on a brake lathe Resurfacing a Disc Typical rotor mounting on a brake lathe

Install a damper to prevent vibration while machining the rotor Resurfacing a Disc Install a damper to prevent vibration while machining the rotor

Brake Lathe This lathe has a large cover to protect operators and others in the shop from flying debris

On-Car Brake Lathe Useful on vehicles where the brake disc is hard to remove or where the manufacturer specifies on-car rotor machining Mounts on the vehicle’s steering knuckle An electric motor turns the disc during the machining process

A stand-mounted electric motor rotates the disc On-Car Brake Lathe A stand-mounted electric motor rotates the disc

On-Car Brake Lathe The lathe assembly has controls for feeding cutters into the disc surface

Disc Brake Reassembly After installing the rotor, fit the caliper assembly into place High-temperature silicone is used on the backs of the pads to help prevent brake rattle and squeal Apply a small amount of silicone grease to the caliper mounting bolt threads

Disc Brake Reassembly Disc brake components

Disc Brake Reassembly

The rotor and lining surfaces should be perfectly clean after repairs Disc Brake Reassembly The rotor and lining surfaces should be perfectly clean after repairs

Drum Brake Service Service is needed anytime your diagnosis finds faulty drum brake components Common drum brake faults: leaking wheel cylinder worn or contaminated linings scored drum seized parking brake components

Spring removal and adjustment tools Tools and Procedures Spring removal and adjustment tools

Tools and Procedures A brake vacuum is being used to prevent inhalation of brake lining dust

Drum Brake Disassembly Drum brake components

Drum Brake Disassembly Remove the wheels and brake drums and, if necessary, back off the adjuster to ease the removal of a worn drum

Drum Brake Disassembly Use a brake spring retracting tool to remove the upper shoe retracting springs Use a hold-down spring tool to remove the hold-down springs

Drum Brake Disassembly Lift the brake shoes off the backing plate Remove the automatic adjuster mechanism Clean the backing plate

Servicing Wheel Cylinders A cylinder must be replaced or rebuilt if it shows signs of leakage or sticking Many shops service the wheel cylinders anytime the linings are replaced Wheel cylinder rebuilding involves honing the cylinder and replacing the rubber cups and boots

Note the wheel cylinder components Drum Brake Assembly Note the wheel cylinder components

Brake Drum Resurfacing Needed when the drum is scored, out-of-round, or worn unevenly Mount the drum on the lathe Wrap a silencing band around the drum Feed the cutting tool against the inner surface of the drum Adjust the depth of the cut to lathe specs and activate the automatic feed

Brake Drum Resurfacing Use the correct collars and hubs to secure the drum to the lathe shaft

Brake Drum Resurfacing This brake lathe has a shoe grinder, which will arc shoes to fit the drum

Measuring Brake Drum Diameter Typically, a drum should not be machined more than 0.060" (1.5 mm) oversize The drum diameter is often stamped on the drum Use a drum micrometer to measure the diameter

Drum Brake Reassembly Clean the wheel bearings Inspect the bearings for wear or damage Pack the bearings with grease and install new grease seals

Drum Brake Reassembly Wipe high-temperature grease on the raised pads on the backing plate and on the star wheel threads

Install the new shoes and the adjuster mechanism on the backing plate Drum Brake Reassembly Install the new shoes and the adjuster mechanism on the backing plate

Preadjusting Drum Brakes Ensures proper initial brake system operation Fit a brake adjusting gauge into the brake drum Set the gauge for the inside diameter of the drum Fit the gauge over the brake shoes Adjust the brake until the linings touch the gauge

Preadjusting Drum Brakes Fitting the gauge into the brake drum Adjusting the shoes

Preadjusting Drum Brakes Another method of preadjusting the brakes: use a brake spoon to turn the star wheel turn the wheel until the brake drum drags slightly when rotated by hand

Parking Brake Adjustment To adjust the parking brake, tighten an adjustment nut on the cable mechanism Shorten the cable until the brakes apply firmly within the first 2/3 of the pedal or lever travel Do not overtighten the brake brakes should not drag when released

Parking Brake Cables