© 2012 Delmar, Cengage Learning Brake Service Chapter 58

© 2012 Delmar, Cengage Learning

Brake Service

Chapter 58


Objectives• Inspect brake systems and recommend needed

repairs• Diagnose brake system problems• Perform brake repairs and adjustments using

the correct materials and procedures


Introduction and Brake Inspection

• This chapter deals with hydraulic brake systems– Inspection

– Diagnosis

– Repair

• Thorough inspection of the braking system– Performed before any repairs

• Start with pedal and master cylinder


Check Brake Pedal Feel• Check the travel of brake pedal

– Apply foot brakes• Should be ample amount of pedal reserve

• Pedal should feel firm, not spongy– Spongy brake pedal indicates air or moisture in

the system• Calling for a brake bleed


Master Cylinder Inspection• Fluid movement past vent port

– Should always be visible when pedal is applied

• Heavy surge from compensating port – Indicates air in system

• Power booster: operating correctly– Sound of air rushing into booster

• Apply the brakes – Test brake lights

• Hydraulic safety switch service– Brake warning lamp operates during braking


Brake Diagnosis and Undercar Checks

• Brakes that pull to one side – Causes: suspension or steering systems

• Brake pedal pulsation – Results when hydraulic pistons move during stop

– Brakes grab cause: oil or grease on lining

– Brake pull: several reasons

• Undercar checks– Perform a visual inspection of brakes with

vehicle raised on a hoist


Disc Brake Inspection• Disc brakes are inspected to see that a sufficient

amount of friction material remains on pads– Remove a front wheel and visually inspect pad

– Inspect the rotor

– Check rotor runout with a dial indicator

– Inspect rotor thickness and brake drums

– Inspect the caliper and wheel seals

– Test parking brake

– Inspect tubing and hoses

– Check for fluid leaks


Inspecting Drum Brake Assemblies

• Important steps– Remove and mark the brake drum

• Spray the flange area with penetrating oil• Use a hammer to rap on drum

– Inspect drum brake cylinders

– Inspect drum brake linings

– Check self-adjuster operation


Brake Fluid Service and Brake Fluid Testing

• Correct fluid to use is listed on reservoir– Space must be left to allow fluid expansion

• Fluid change interval– Change fluid every two years or 30,000 miles

• Brake fluid testing methods– Moisture content testers

– Refractometer testing

– Voltmeter testing

– Fluid test strips


Bleeding Brakes and Brake Bleeding Methods

• Air or moisture in system can result in soft pedal– Brakes are bled through a bleed screw

• Manufacturers specify bleeding sequences

• Brake bleeding methods– Manual bleeding and manual bleeding with a hose

– Pressure bleeding and vacuum bleeding

– Reverse fluid injection and gravity bleeding

• Several problems can occur during bleeding• Scan tool to may be required to cycle the ABS• Avoid contamination with petroleum products


Adjusting Brakes• Improper brake adjustments

– Cause a low, firm pedal

• Diagnosis– Pump pedal twice quickly to correctly diagnose

• Pedal height rises higher on second application: brakes need adjustment

– Most brakes today have self-adjusting mechanisms


Master Cylinder Service and Master Cylinder Removal

• Cover vent is obstructed– Air can be drawn in at back of master cylinder,

aerating fluid• Air in fluid causes a spongy pedal

– Check the vent when checking fluid level

• Removing master cylinder– Be sure to use fender covers

– Use a vacuum brake bleeder to empty fluid from master cylinder reservoir

• Then remove master cylinder


Master Cylinder Disassembly• Master cylinders

can be purchased as new or rebuilt– Bore of a cylinder

is corroded or pitted: cylinder must be replaced

• Quick take-up master cylinders are serviced in the same manner


Bench Bleeding the Master Cylinder

• Before installing a master cylinder– Fill with fluid and bleed air

• Quick take-up cylinder– Casting is larger near the rear

• Pedal free travel – Checked when master cylinder replaced

• Should be less than 1/8” of free play


Brake Job• Front or rear linings are replaced in pairs

– Consult the appropriate service manual

• Complete brake job includes:– All internal hydraulic parts of wheel cylinders and

disc calipers

– New brake fluid, hardware, and springs

– Drums and rotors may be remachined on lathe

• Ethics in brake work– Include disclaimer when incomplete brake job is

advertised


Drum Brake Lining Removal• Clean entire assembly before disassembling

brakes– Brake dust is dangerous to breathe

• Low-pressure wet brake washers are the most popular way of cleaning brake assemblies

• HEPA vacuum can also be used

• Removal of brake linings – Requires special tools


Rebuilding Hydraulic Cylinders• Hydraulic wheel cylinders and disc brakes can

be rebuilt if made of cast iron and not corroded– Two kinds of hone are available

• After honing: install new rubber parts


Reassembling a Wheel Cylinder and Removing Wheel Cylinders• Reassembling a wheel cylinder

– Lips on the wheel cylinder cups face toward fluid

– Use brake fluid liberally as an assembly lubricant

• Removing wheel cylinders– Pitted or corroded: must be replaced

– Use flare-nut wrench to remove brake tubing fitting

– Cylinder is held to backing by two screws or clip • Requires special service tool


Replacing Drum Brake Shoes• Important steps

– Clean backing plates• Use high-temperature lubricant on pads

– Inspect brake springs• Shoe return springs should not be loose or broken

– Service self-adjusters • Must be installed on the correct side of the car


Adjusting Drum Brake Clearance

• Initial clearance adjustment – Made before drums installed

• Brake adjusting gauge – Adjusted to size of drum

• Star wheel of adjuster – Turned until shoes expand to size of adjusting

gauge

• Manual adjustment – Can be done while the drum is installed


Drum and Rotor Service• Drum or disc lining

are replaced– Common practice:

remachine drums or rotors

• Too much metal must be removed: replace


Drum Service• Maximum amount that can be cut from most

drums is 0.060”– Typical larger drum lists a discard diameter that

is 0.090” larger than original

• Important points– Inspect drums to see that they are not out-of-

round or scored

– Be sure there is no oil or grease on drum

– Drums and rotors can be mounted on a lathe

– Tool bit at end of the cutter is a carbide insert


Rotor Service• Rotor in good condition

– Provides better surface than freshly machined • Rotors are always machined in pairs

• Machining a rotor– Cut both sides at once, maintaining equal force

on both sides

– Newly machined surfaces must be cleaned


On-Vehicle Rotor Machining• On-the-car brake lathes: becoming more popular

– Before reinstalling a brake rotor: clean any rust preventive material from surface

– Use a torque wrench when tightening lug nuts


Disc Brake Service• Important points

– Disc linings are usually easy to replace

– Most caliper designs require removal of caliper to remove pads

– Before you install replacement pads in a floating caliper, check the condition of its slides

– Open the bleed screw before retracting the piston

– All pistons must be compressed at the same time


Rear Disc Pad Installation and Rebuilt Calipers

• Rear disc pad installation – Rear-wheel disc brakes have a built-in parking

brake

– If there is a lever, do not force the piston into the bore

• Rebuilt calipers– Many shops find it more cost-effective to install

rebuilt calipers• Unloaded and loaded


Disc Caliper Rebuilding• Major steps

– Disassemble caliper

– Clean and inspect caliper parts

– Inspect the pistons

– Reassembly caliper

– Install disc pads in the caliper


Selecting Brake Linings• Various lining materials are available

– Depends on application and friction characteristics desired

• Older brake lining materials– Classified as organic or inorganic

• Side edge of a new brake lining – Stamped with code number established by SAE

• Aftermarket friction material certifications– Assure aftermarket lining performance

characteristics equal to or better than new cars


Disc Brake Noise• Brake noise and vibration

– Most likely complaints from a customer

– Important that disc linings be firmly attached

– Most pads have anti-rattle clips

– Metal-to-metal contact points must be lubricated to prevent noise

– Install caliper on correct side

– Today’s linings are more fully cured but still require some break-in


Parking Brake Cable Service• During a rear brake job

– Disconnect emergency brake cables from brake linings

– Wipe off cables and apply clean grease

• Brake cable requires replacement– Use small hose clamp to remove it from backing

plate

• Brake should be fully applied at half travel– Follow manufacturer’s recommendations


Vacuum Power Brake Service• Repairs to power boosters include replacement

of the hose, filter, or check valve• Causes of increased pedal effort

– Installation of larger diameter tires

– Defective power booster

• Vacuum booster operation test – Exhaust all vacuum reserve from power booster

• Power brake booster problems – Can be due to a hole in the booster diaphragm or

a stuck valve that is leaking


Vacuum Power Brake Service (cont’d.)

• Defective power booster – Causes brakes to drag

• Vacuum supply checks– Check hose that supplies vacuum to the power

booster from intake manifold

• Power brake booster problems – Hole in booster diaphragm or a stuck valve

• Typical power boosters – Have four studs that protrude through the

bulkhead into the passenger compartment


Brake Warning Lamp Diagnosis• Hydraulic safety switches

– Found on vehicles that do not have fluid level sensing systems

• Fluid level switches – Found on vehicles that do not have hydraulic

safety switch

• Check operation of stoplight switch– Apply brakes and verify that stoplights come on

– Use a wiring diagram to determine how the circuit operates


Antilock Brake System (ABS) System Service

• Covered in Chapter 59

Documents

© 2012 Delmar, Cengage Learning Brake Service Chapter 58