Brake Fundamentals and Repair

Chapter 57

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 • Diagnose and repair braking systems

• Brake linings may contain Asbestos

Brake Safety

• Components maybe Hot!!!

• Brakes may be held together with springs • Thing may be spinning

Intro Automotive Brakes

• When a car is moving, it has kinetic energy (inertia) • Brakes uses friction to stop, or hold the vehicle • Brakes convert mechanical (moving) energy into heat Up to 600 degrees

Intro cont.

• Coefficient of friction varies with: – Temperature – Rubbing speed – Condition of surfaces • During stop, vehicle weight shifts to front brakes Up to 80%

Brake system layout

Systems ID

-

Front/rear split

found on rear-wheel-drive vehicles -

Diagonal split

found on FWD and high front-to-rear brake ratio -Vehicles with ABS each wheel can have its own circuit

Disc Brakes

• Caliper – applies the brakes using hydraulic pressure • Brake pads- contact the inboard and outboard of the rotor • Rotors- a disc that connects to the hub, used to stop the wheel

Brake Linings

• • • • Linings are bonded or riveted backing plate

Semimetallic

– sponge iron and steel fibers

Metallic

– heavy-duty and racing conditions

Ceramic

– ceramic and copper fibers to control heat

Rotor parts ID

•

Inboard surface

pad surface – •

Rotor hat

– is the metal that connects the flange to the rotor •

Mounting flange

– slides on or bolts to the hub

Rotor parts ID cont.

• Hub rotors – contains bearing races • Captured rotors – bolt the back of the hub

Rotor Design

•

Vented rotors

Vents help circulate air to dissipates heat better - Found on the front and rear brakes •

Non vented rotors

– Used mostly on rears – Light duty applications

Measuring rotors

• Measure the thickness of the rotor • Measure the lateral runout of the rotor

Spec Sheet

FRONT

Maximum lateral runout 0.003 in Thickness Variation 0.0005 in Maximum Scoring 0.060 in Discard Thickness 1.210 in Minimum Rotor Thickness 1.220 in New Rotor Thickness 1.270 in

REAR

Rotor Discard Thickness 0.350 in Minimum Rotor Thickness 0.390 in New Rotor Thickness 0.430 in

Brake calipers

• Applied by hydraulic pressure • Squeeze the pads onto the rotor • Self adjusting

Brake caliper parts ID

Caliper seal design

•

Square cut seal

– Pulls the piston back after applied – Allows the piston to self adjust – Keeps fluid in the caliper

Floating caliper

– Caliper body slides on sliders – Has 1 or 2 pistons only on one side – Light to moderate braking force

Non Floating Caliper

- Has 4 pistons or more - Equal number of pistons per side - Used for moderate to heavy braking - Pistons provide the movement

Measuring brake pads

• Can be measured with a caliper or a plastic gauge.

• Wear indicators alert the drive of excessive wear.

Machining Brake rotors

• On car Brake lathes • Bench Brake lathes

Drum Brakes

• Braking system is enclosed • Found on rear axles • Great initial stopping power

Drum Brake types

•

Leading – trailing

- Anchor on bottom •

Duo servo

-Anchor on top

Drum brake components

•

Brake shoes

- Friction material are riveted or bonded steel plate - Friction material can range in length

On most dual-servo brake primary shoes, the lining is positioned near the center of the lining table. Higher or lower lining positions provide better braking action, or prevent noise, in certain applications.

•

Shoe lining position

- Higher position better action - lower position for noise prevention

Components cont.

• • • •

Lining Material’s Semi-metallic

– sponge iron and steel fibers

Metallic

– heavy-duty and racing conditions

Ceramic

– ceramic and copper fibers to control heat

Component cont.

• Lining codes in three groups - 1 st manufacture name and code - 2 nd is the friction code “cold” - 3 rd is the friction code “hot”

Friction chart

These codes were established by the SAE (Society of Automotive Engineers): Code C Code D 0.00 to 0.15

0.15 to 0.25

Code E Code F Code G 0.25 to 0.35

0.35 to 0.45

0.45 to 0.55

Code H 0.55 and above Code Z un-graded

Backing plate

• All components attach to it “except drum” • Mounts to the axle • Brakes apply against it

component cont.

•

Hold down springs

- varies in shape - compression spring - uses pins through the backing plate - uses small retainers - parts get fatigued

Components cont

•

Return spring

- various lengths - various tensions - retracts shoes back to the rest position - High tension

Components cont.

•

Uni-spring

- Found on leading trailing systems - Connects to the anchor on the backing plate - is a hold-down and return spring

Component cont.

•

Adjusters -

composites for shoe wear

-

attaches to both shoes

-

Are know for seizing

Duo servo adjusters linkage

•

Cable type

- runs along secondary shoe - attaches from anchor to the adjuster pawl - adjusts when brakes are released

Duo servo adjusters linkage

•

lever type

- runs along secondary shoe - pivots on hold down pin - adjusts when brakes are applied

Leading trailing adjuster linkage

•

Leading adjuster pawl

- Arm attaches on primary shoe - adjusts when brakes are applied - uses a designated spring

Leading trailing adjuster linkage

• Trailing adjuster pawl - Arm is located on secondary shoe - adjusts when brakes are released - uses return spring

Wheel cylinders

• Applies the rear shoes equally • Cups are usually the reason of failure • Bleeder must be installed on top • Secured by bolts or clips

Brake hose

• Provides flexibility for suspension system • Consists of hard plastic line wrapped woven fabric and rubber • Failures - dry cracked - clasps internally - internal leak causing a bubble

Banjo Bolt

• The bolt is hollow • Seals with copper washers • Washers should not be reused • Bolts break easily

Brake Lines

• Transfer fluid pressure from the master cylinder to the wheel brake assemblies • Lines – made of double wall steel tubing – made of seamless construction – tubing ends usually have double-lap flares or bubble flares

There are two types of double-walled tubing: Seamless Multiple ply

All double-walled brake tubing is plated with tin, zinc, or other similar substances for protection against rust and corrosion.

CAUTION:

Copper tubing should

never

be used for brake lines. Copper tends to burst at a lower pressure than steel.

All steel brake lines have one of two basic types of ends: Double Flare.

ISO, International Standards Organization (also called a ball flare or bubble flare).

Combination Valve

• Multiple functions in one Assembly • designs depend on braking demands • modern vehicle the these valves incorporated in the EBCM

Metering valve

• Found on front disc and rear drum vehicles • Allows the rear brakes engage before front • Overcomes drum return spring pressure • Allows for more stable braking

Warning system

• Uses a spool valve between the front and rear circuits • Valve will move to side with the lowest pressure • When moved grounds the switch and turns on the warning light • Must be reset before bleeding

Proportioning valve

• Front to rear split system • Limits the rear brake applying pressure.

• Uses a spring and stem valve

Master Cylinders

Master cylinders types

• Front to rear split – Two ports – Found in trucks larger cars • Diagonal split – Four ports – Found in FWD cars – If front circuit fails, rears are not adequate

Master Cylinder Reservoirs

Reservoir,

• Housing can by plastic or casted • Fluid level sensor • Reservoir seals – Vented between the cap and seal to allow fluid level change

Fluid Ports

Brakes unapplied

Brakes applied

•Primary cup seal slides over vent port •Pressure builds in primary and pushes secondary •Secondary slides forward blocking vent port •Secondary builds pressure

Quick take-up master cylinders

• Introduced in newer vehicles • Gives the ability to use low drag calipers • Made of aluminum or composite material

At

70 to 100 psi

, the check ball valve in the quick take-up valve allows fluid to return to the brake fluid reservoir. Because the quick take-up "works" until 100 psi is reached, a metering valve is not required to hold back the fluid pressure to the front brakes.

Master cylinder failures

• Fluid leak from push rod end • Seal failure • Level sensor failure • Inability to bleed brakes – Push rod length – Valve position

PASCAL'S LAW

The hydraulic principles that permit a brake system to function were discovered by a French physicist, Blaise Pascal (1632 1662).

Pascal’s Law

states that "when force is applied to a liquid confined in a container or an enclosure, the pressure is transmitted equal and undiminished in every direction.“ Since this

force

measured in pounds (lb.) or Newtons (N) is applied to a piston with an area measured in square inches, the

pressure

is the force divided by the area or "10 pounds per square inch" (psi). It is this "pressure" that is transmitted, without loss, throughout the entire hydraulic system.

In the simple brake system, the pedal and linkage apply a 100-pound force on a master cylinder piston with an area of 1 square inch.

MASTER CYLINDER OPERATION

Brake pedal movement and force are transferred to the brake fluid and directed to wheel cylinders or calipers

.

Vacuum Brake Boosters

Vacuum boosters • Uses vacuum from the engine or a vacuum pump • Most common type of assist unit • when applied can produce 200 to 300 psi

• Air valve in back position • Allows vacuum to enter primary section • vacuum control port closed to atmosphere Brakes unapplied

• Air valve blocks vacuum passage

Brakes applied

• vacuum control port (opens) • Atmospheric pressure enters primary side •Floating control valve and reaction disk bleed off pressure to stabilize

Vacuum booster tests and failures • Engine off reserve assist 3 to 4 brake depressed - check valve • Pump the booster then start engine, pedal should drop • No brake assist, engine idea rises or lowers w/ depressed - loud hissing noise • engine sucking in brake fluid

Power Master Power Brake Unit

• Nitrogen charged • Electric 12V pump • Accumulator contains 510 to 685 psi • Pressure sensor turns pump on

Power Master Diagnosis

• Pressure gauge inline of pressure sensor •No residual pressure after long period of time •

NEVER TAKE APART A POWER MASTER UNIT!!!

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER

Unapplied fluid flow

• Hydraulic pressure enters the spool valve then out to steering gear • Spool valve is in the rest position • passage to power chamber is closed

Brake pedal depressed

• Spool valve moved • Blocks off return line passage • Fluid flows through the spool valve • Pressure builds behind the power piston • Reaction rod controls the apply pressure

Accumulator

• Fills during the brake apply •

Caution

internally large spring • Contains residual pressure in case of engine failure

Hydro boost testing

• No steering or brake assist. “pump concern” • Fluid level • Fluid leaks • Internal assist unit

Emergency Brakes

• Appling Mechanism

Cable system

• One cable comes from handle • Equalizer makes both rear apply equally • E-brakes are always going to the rear • All assemblies have a tread adjuster

Drum brake –E-brake mech.

• Cable secures to backing plate • Pulls on parking brake lever • Lever pivots on shoe • Strut travels from lever to other shoe

E- brake drum w/rear disc

• Uses the hat of the rotor as the drum • Small shoes internally • Applies mechanically

Caliper E- brake systems

• Cable connects to a back of the calper