Dodge Durango (DN). Manual — part 239

depressed. The speed control and brake sensor cir-
cuits is closed until the plunger is depressed.

RED BRAKE WARNING LAMP

DESCRIPTION

A red warning lamp is used for the service brake

portion of the hydraulic system. The lamp is located
in the instrument cluster.

OPERATION

The red warning light alerts the driver if a pres-

sure differential exists between the front and rear
hydraulic systems or the parking brakes are applied.
The lamp is turned on momentarily when the igni-
tion switch is turn to the on position. This is a self
test to verify the lamp is operational.

POWER BRAKE BOOSTER

DESCRIPTION

All models use a tandem diaphragm, power brake

booster.

NOTE: The power brake booster is not a repairable
component. The booster must be replaced as an
assembly if diagnosis indicates a malfunction has
occurred.

OPERATION

The booster unit consists of a single housing

divided into two by a tandem diaphragm. The outer
edge of the diaphragm is secured to the housing. The
booster push rod, which connects the booster to the
brake pedal and master cylinder, is attached to the
center of the diaphragm. A check valve is used in the
booster outlet connected to the engine intake mani-
fold. Power assist is generated by utilizing a combi-
nation of vacuum and atmospheric pressure to boost
brake assist.

MASTER CYLINDER

DESCRIPTION

A two-piece master cylinder is used on all models.

The cylinder body containing the primary and sec-
ondary pistons is made of aluminum. The removable
fluid reservoir is made of nylon reinforced with glass
fiber. The reservoir stores reserve brake fluid for the
hydraulic brake circuits. The reservoir is the only
serviceable component.

The fluid compartments of the nylon reservoir are

interconnected to permit fluid level equalization.
However, the equalization feature does not affect cir-
cuit separation in the event of a front or rear brake

malfunction. The reservoir compartments will retain
enough fluid to operate the functioning hydraulic cir-
cuit.

Care must be exercised when removing/installing

the master cylinder connecting lines. The threads in
the cylinder fluid ports can be damaged if care is not
exercised. Start all brake line fittings by hand to
avoid cross threading.

The cylinder reservoir can be replaced when neces-

sary. However, the aluminum body section of the
master cylinder is not a repairable component.

NOTE: If diagnosis indicates that an internal mal-
function has occurred, the aluminum body section
must be replaced as an assembly.

OPERATION

The master cylinder bore contains a primary and

secondary

piston.

The

primary

piston

supplies

hydraulic pressure to the front brakes. The secondary
piston supplies hydraulic pressure to the rear brakes.

COMBINATION VALVE

The combination valve contains a pressure differ-

ential valve and switch and a proportioning valve.
The combination valve/proportioning valve are not
repairable and must be replaced as an assembly.

PRESSURE DIFFERENTIAL SWITCH

The pressure differential switch is connected to the

brake warning lamp. The switch is triggered by
movement of the switch valve. The purpose of the
switch is to monitor fluid pressure in the separate
front/rear brake hydraulic circuits.

A decrease or loss of fluid pressure in either

hydraulic circuit will cause the switch valve to shut-
tle forward or rearward in response to the pressure
differential. Movement of the switch valve will push
the switch plunger upward. This closes the switch
internal contacts completing the electrical circuit to
the warning lamp. The switch valve may remain in
an actuated position until repair restores system
pressures to normal levels.

PROPORTIONING VALVE

The proportioning valve is used to balance front-

rear brake action at high decelerations. The valve
allows normal fluid pressure during moderate brak-
ing. The valve only controls fluid pressure during
high decelerations brake stops, when a percentage of
rear weight is transferred to the front wheels.

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BRAKES

5 - 3

DESCRIPTION AND OPERATION (Continued)

FRONT DISC BRAKES

DESCRIPTION

The calipers are a single piston type. The calipers

are free to slide laterally, this allows continuous com-
pensation for lining wear.

OPERATION

When the brakes are applied fluid pressure is

exerted against the caliper piston. The fluid pressure
is exerted equally and in all directions. This means
pressure exerted against the caliper piston and
within the caliper bore will be equal (Fig. 1).

Fluid pressure applied to the piston is transmitted

directly to the inboard brake shoe. This forces the
shoe lining against the inner surface of the disc
brake rotor. At the same time, fluid pressure within
the piston bore forces the caliper to slide inward on
the mounting bolts. This action brings the outboard
brake shoe lining into contact with the outer surface
of the disc brake rotor.

In summary, fluid pressure acting simultaneously

on both piston and caliper, produces a strong clamp-
ing action. When sufficient force is applied, friction
will attempt to stop the rotors from turning and
bring the vehicle to a stop.

Application and release of the brake pedal gener-

ates only a very slight movement of the caliper and
piston. Upon release of the pedal, the caliper and pis-
ton return to a rest position. The brake shoes do not
retract an appreciable distance from the rotor. In
fact, clearance is usually at, or close to zero. The rea-
sons for this are to keep road debris from getting
between the rotor and lining and in wiping the rotor
surface clear each revolution.

The caliper piston seal controls the amount of pis-

ton extension needed to compensate for normal lining
wear.

During brake application, the seal is deflected out-

ward by fluid pressure and piston movement (Fig. 2).
When the brakes (and fluid pressure) are released,
the seal relaxes and retracts the piston.

The amount of piston retraction is determined by

the amount of seal deflection. Generally the amount
is just enough to maintain contact between the pis-
ton and inboard brake shoe.

DRUM BRAKES

DESCRIPTION

Drum brakes on all models are dual shoe, internal

expanding units with an automatic self adjusting
mechanism (Fig. 3). Nine inch and eleven inch
brakes are used.

OPERATION

When the brake pedal is depressed hydraulic pres-

sure pushes the rear wheel cylinder pistons outward.
The wheel cylinder push rods then push the brake
shoes outward against the brake drum. When the
brake pedal is released return springs attached to
the brake shoes pull the shoes back to there original
position.

Fig. 1 Brake Caliper Operation

1 – CALIPER
2 – PISTON
3 – PISTON BORE
4 – SEAL
5 – INBOARD SHOE
6 – OUTBOARD SHOE

Fig. 2 Lining Wear Compensation By Piston Seal

1 – PISTON
2 – CYLINDER BORE
3 – PISTON SEAL BRAKE PRESSURE OFF
4 – CALIPER HOUSING
5 – DUST BOOT
6 – PISTON SEAL BRAKE PRESSURE ON

5 - 4

BRAKES

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DESCRIPTION AND OPERATION (Continued)

PARKING BRAKES

DESCRIPTION

The rear drum brake shoes serve as the parking

brakes. The parking brakes are operated by a system
of cables and levers attached to the rear brake sec-
ondary shoes.

OPERATION

The shoes make contact with the brake drum sur-

face by a cable and lever mechanism attached to the
secondary brake shoe. The front parking brake cable
is connected to the parking brake pedal and to the
rear cables. An intermediate cable is used on some
vehicles to connect the front and rear cables.

The parking brake pedal assembly is mounted on

the driver side cowl panel. The front cable is directly
attached to the assembly. The pedal assembly con-

tains a spring loaded mechanism to hold the pedal in
the applied position. A rod and spring are used to
release the ratchet mechanism and return the pedal
to normal position.

BRAKE HOSES AND LINES

DESCRIPTION

Flexible rubber hose is used at both front brakes

and at the rear axle junction block. Double walled
steel tubing is used to connect the master cylinder to
the major hydraulic braking components and then to
the flexible rubber hoses. Double inverted style and
ISO style flares are used on the brake lines.

OPERATION

The hoses and lines transmit the brake fluid

hydraulic pressure to the calipers and or wheel cyl-
inders.

DIAGNOSIS AND TESTING

BASE BRAKE SYSTEM

Base brake components consist of the brake shoes,

calipers, wheel cylinders, brake drums, rotors, brake
lines, master cylinder, booster, and parking brake
components.

Brake diagnosis involves determining if the prob-

lem is related to a mechanical, hydraulic, or vacuum
operated component.

The first diagnosis step is the preliminary check.

PRELIMINARY BRAKE CHECK

(1) Check condition of tires and wheels. Damaged

wheels and worn, damaged, or underinflated tires
can cause pull, shudder, vibration, and a condition
similar to grab.

(2) If complaint was based on noise when braking,

check suspension components. Jounce front and rear
of vehicle and listen for noise that might be caused
by loose, worn or damaged suspension or steering
components.

(3) Inspect brake fluid level and condition. Note

that the brake reservoir fluid level will decrease in
proportion to normal lining wear. Also note that
brake fluid tends to darken over time. This is
normal and should not be mistaken for contam-
ination.

(a) If fluid level is abnormally low, look for evi-

dence of leaks at calipers, wheel cylinders, brake
lines, and master cylinder.

(b) If fluid appears contaminated, drain out a

sample to examine. System will have to be flushed
if fluid is separated into layers, or contains a sub-
stance other than brake fluid. The system seals

Fig. 3 Brake Assembly

1 – SUPPORT PLATE
2 – RETURN SPRING
3 – ANCHOR PLATE
4 – RETURN SPRING
5 – CABLE GUIDE
6 – PARKING BRAKE LEVER
7 – ADJUSTER CABLE AND SPRING
8 – SHOE RETAINER, SPRING AND PIN
9 – SECONDARY SHOE AND LINING
10 – LEVER SPRING
11 – ADJUSTER LEVER
12 – ADJUSTER SCREW ASSEMBLY
13 – SHOE SPRING
14 – PRIMARY SHOE AND LINING
15 – PARKING BRAKE STRUT AND SPRING

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BRAKES

5 - 5

DESCRIPTION AND OPERATION (Continued)

and cups will also have to be replaced after flush-
ing. Use clean brake fluid to flush the system.
(4) Check parking brake operation. Verify free

movement and full release of cables and pedal. Also
note if vehicle was being operated with parking
brake partially applied.

(5) Check brake pedal operation. Verify that pedal

does not bind and has adequate free play. If pedal
lacks free play, check pedal and power booster for
being loose or for bind condition. Do not road test
until condition is corrected.

(6) Check booster vacuum check valve and hose.
(7) If components checked appear OK, road test

the vehicle.

ROAD TESTING

(1) If complaint involved low brake pedal, pump

pedal and note if it comes back up to normal height.

(2) Check brake pedal response with transmission

in Neutral and engine running. Pedal should remain
firm under constant foot pressure.

(3) During road test, make normal and firm brake

stops in 25-40 mph range. Note faulty brake opera-
tion such as low pedal, hard pedal, fade, pedal pulsa-
tion, pull, grab, drag, noise, etc.

(4) Attempt to stop the vehicle with the parking

brake only and note grab, drag, noise, etc.

PEDAL FALLS AWAY

A brake pedal that falls away under steady foot

pressure is generally the result of a system leak. The
leak point could be at a brake line, fitting, hose, or
caliper/wheel cylinder. If leakage is severe, fluid will
be evident at or around the leaking component.

Internal leakage (seal by-pass) in the master cylin-

der caused by worn or damaged piston cups, may
also be the problem cause.

An internal leak in the ABS or RWAL system may

also be the problem with no physical evidence.

LOW PEDAL

If a low pedal is experienced, pump the pedal sev-

eral times. If the pedal comes back up worn linings,
rotors, drums, or rear brakes out of adjustment are
the most likely causes. The proper course of action is
to inspect and replace all worn component and make
the proper adjustments.

SPONGY PEDAL

A spongy pedal is most often caused by air in the

system. However, thin brake drums or substandard
brake lines and hoses can also cause a spongy pedal.
The proper course of action is to bleed the system,
and replace thin drums and substandard quality
brake hoses if suspected.

HARD PEDAL OR HIGH PEDAL EFFORT

A hard pedal or high pedal effort may be due to

lining that is water soaked, contaminated, glazed, or
badly worn. The power booster or check valve could
also be faulty.

PEDAL PULSATION

Pedal pulsation is caused by components that are

loose, or beyond tolerance limits.

The primary cause of pulsation are disc brake

rotors with excessive lateral runout or thickness vari-
ation, or out of round brake drums. Other causes are
loose wheel bearings or calipers and worn, damaged
tires.

NOTE: Some pedal pulsation may be felt during
ABS activation.

BRAKE DRAG

Brake drag occurs when the lining is in constant

contact with the rotor or drum. Drag can occur at one
wheel, all wheels, fronts only, or rears only.

Drag is a product of incomplete brake shoe release.

Drag can be minor or severe enough to overheat the
linings, rotors and drums.

Minor drag will usually cause slight surface char-

ring of the lining. It can also generate hard spots in
rotors and drums from the overheat-cool down pro-
cess. In most cases, the rotors, drums, wheels and
tires are quite warm to the touch after the vehicle is
stopped.

Severe drag can char the brake lining all the way

through. It can also distort and score rotors and
drums to the point of replacement. The wheels, tires
and brake components will be extremely hot. In
severe cases, the lining may generate smoke as it
chars from overheating.

Common causes of brake drag are:
• Seized or improperly adjusted parking brake

cables.

• Loose/worn wheel bearing.

• Seized caliper or wheel cylinder piston.

• Caliper binding on corroded bushings or rusted

slide surfaces.

• Loose caliper mounting.

• Drum brake shoes binding on worn/damaged

support plates.

• Mis-assembled components.

• Long booster output rod.
If brake drag occurs at all wheels, the problem

may be related to a blocked master cylinder return
port, or faulty power booster (binds-does not release).

5 - 6

BRAKES

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DIAGNOSIS AND TESTING (Continued)