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OBJECTIVESAfter studying Chapter 6, the reader
should be able to1. Prepare for the Brakes
(A5) ASE certification test.2. List the types
of brake fluids.3. Describe where armored brake
line is used.4. Discuss the differences between
double flare and ISO flare.5. Explain how
flexible brake lines should be handled during
service.6. List the precautions necessary when
handling or disposing of brake fluid.7. Discuss
the types of rubber that are used in brake system
components.
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BRAKE FLUIDBrake fluid boiling point is one of
the most critical aspects and ratings for brake
fluid. Brake fluid is hygroscopic, meaning that
it readily absorbs water. As brake fluid ages, it
absorbsmoisture, which lowers its boiling point
and causes increased corrosion of the brake
system components.
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BRAKE FLUID TYPESBrake fluid is a
polyalkyleneglycolether mixture called
polyglycol for short. All polyglycol brake fluid
is clear to amber in color. Brake fluid has to
have the following characteristicsA high
boiling pointA low freezing pointNo ability to
damage rubber parts in the brake system
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BRAKE FLUID SPECIFICATIONSAll automotive brake
fluid must meet federal Motor Vehicle Safety
Standard 116. The Society of Automotive Engineers
(SAE) and the Department of Transportation (DOT)
have established brake fluid specification
standards.
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DOT 3 DOT 4 DOT 5.1 DOT 5
Dry boiling point Dry boiling point Dry boiling point Dry boiling point Dry boiling point
F 401 446 500 500
C 205 230 260 260
Wet boiling point Wet boiling point Wet boiling point Wet boiling point Wet boiling point
F 284 311 356 356
C 140 155 180 180
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DOT 3DOT 3 brake fluid is the type most often
used.1. DOT 3 absorbs moisture. According to
SAE, DOT 3 can absorb 2 of its volume in water
per year. Moisture is absorbed by the brake fluid
through microscopic seams in the brake system and
around seals. Over time, the water will corrode
the system and thicken the brake fluid. The
moisture can also cause a spongy or no brake
pedal if is heated to the point of boiling.
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2. DOT 3 must be used from a sealed (capped)
container. If allowed to remain open for any
length of time, DOT 3 will absorb moisture from
the surrounding air. If the can has been open for
a length of time, use a new can of
fluid.CAUTION  DOT 3 brake fluid is a very
strong solvent and can remove paint! Care is
required when working with DOT 3 brake fluid to
avoid contact with the vehicles painted
surfaces. It also takes the color out of leather
shoes.
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DOT 4This brake fluid is formulated for use by
all vehicles, imported or domestic. DOT 4 is
polyglycol based but has borate esters added to
provide an extra buffer for the fluid against
acids that can form in the moisture that has been
absorbed in the fluid when it is heated. DOT 4
can often be used where DOT 3 is used and even
though the two types of brake fluid are
compatible and miscible (able to be mixed), some
vehicle manufacturers recommend that DOT 3 and
DOT 4 not be mixed.
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DOT 5.1DOT 5.1 is a nonsilicone-based
polyglycol fluid and is clear to amber in color.
This severe duty fluid has a boiling point of
over 500ºF, equal to the boiling point of
silicone-based DOT 5 fluid. Unlike DOT 5, DOT
5.1 can be mixed with either DOT 3 or DOT 4,
according to brake fluid manufacturers
recommendations.CAUTION Some vehicle
manufacturers do not recommend the use of or the
mixing of other types of polyglycol brake fluid
and specify the use of DOT 3 brake fluid only.
Always follow the vehicle manufacturers
recommendation.
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DOT 5DOT 5 brake fluid is commonly called
silicone brake fluid and is made from
polydimethylsiloxanes. It does not absorb any
water, and is therefore called nonhygroscopic.
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DOT 5 brake fluid is purple (violet) in color to
distinguish it from DOT 3 or DOT 4 brake fluid.
Silicones have about three times the amount of
dissolved air as glycol fluids (about 15 of
dissolved air versus only about 5 for standard
glycol brake fluid).
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1. Silicone brake fluid has an affinity for air
therefore, it is more difficult to bleed the
hydraulic system of trapped air.2. The trapped
air expands with increasing temperature. This
causes the brake pedal to feel mushy because
the pressure exerted on the hydraulic system
simply compresses the air in the system and does
not transfer the force to the wheel cylinders and
calipers as it should.3. The air trapped in the
silicone brake fluid can also off-gas at high
altitudes, causing a mushy brake pedal and
reduced braking performance.4. DOT 5 brake
fluid should not be mixed with any other type of
brake fluid.5. DOT 5 does not affect rubber
parts and will not cause corrosion.6. DOT 5 is
expensive.
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Old brake fluid (four years old) often has a
boiling point under 300F (150C). With the air
temperature near 100F (38C), it does not take
much more heat to start boiling the brake fluid.
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A tester that can be used to measure the boiling
point of brake fluid.
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Mineral oil causes the rubber compounds that are
used in glycol brake fluid systems to swell.
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To help prevent hydraulic system mineral oil from
being mixed with glycol brake fluid, hydraulic
mineral oils are green.
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BRAKE FLUID INSPECTION AND TESTINGThe brake
fluid should be inspected regularly, including
the following items.1. Proper Level.2.
Color/Condition.
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3. Tested Using a Tester or Test Strips.
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4. Brake Fluid Contamination Test.
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BRAKE FLUID SERVICE PROCEDURES AND PRECAUTIONS1.
Store brake fluid only in its original
container.2. Before opening a brake fluid
container, remove any dirt, moisture, or other
contamination from the top and outside of the
container.3. When a brake fluid container is
empty, it should be discarded4. Do not transfer
brake fluid to any other container that may have
contained oil, kerosene, gasoline, antifreeze,
water, cleaners, or any other liquids or
chemicals.5. Do not reuse brake fluid that has
been siphoned from another vehicle or drawn out
during a brake bleeding operation.6. Use only
fresh new brake fluid for flushing the hydraulic
brake system.
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BRAKE FLUID HANDLING AND DISPOSALBrake fluid
spilled on the floor should be cleaned up using
absorbent material and the material disposed of
in the regular trash. Brake fluid becomes a
hazardous waste if spilled onto open ground,
where it can seep into groundwater.
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RUBBER TYPESVehicles use a wide variety of
rubber in the braking system, suspension system,
steering system, and engine. Rubber products are
called elastomers. Some are oil- and
grease-resistant elastomers and can be harmed by
brake fluid, while others are brake-fluid
resistant and can swell or expand if they come in
contact with oil or grease.
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BRAKE LINESHigh-pressure double-walled steel
brake lines or high-strength flexible lines are
used to connect the master cylinder to each
wheel. The steel brake lines are also called
brake pipes or brake tubing.
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There are two types of double-walled
tubingSeamlessMultiple ply
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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.
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All steel brake lines have one of two basic types
of ends Double Flare.
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ISO, which means International Standards
Organization (also called a ball flare or bubble
flare).
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When replacing steel brake line, new steel tubing
should be used and a double lap flare or an ISO
flare completed at each end using a special
flaring tool.
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Brake line can also be purchased in selected
lengths already correctly flared. They are
available in different diameters, the most
commonly used being 3/16 in. (4.8 mm), 1/4 in.
(6.4 mm), and 5/16 in. (7.9 mm) outside diameter
(O.D.).
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Brake line diameter is also very important and
replacement lines should be the same as the
original. Many vehicle manufacturers use
larger-diameter brake lines for the rear brakes
because the larger line decreases brake response
time. Response time is the amount of time between
the pressure increase at the master cylinder and
the pressure increase at the brakes.
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Always use two line wrenches when disconnecting
or reattaching brake lines.
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COILED BRAKE LINESteel brake line is often
coiled, as used on some Chevrolet/GMC trucks.
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The purpose of the coils is to allow movement
between the brake components without stress that
could lead to metal fatigue and brake line
breakage.
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ARMORED BRAKE LINEIn many areas of the brake
system, the steel brake line is covered with a
wire coil wrap.
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To bend brake line without a tubing bender tool,
use an old V-belt pulley.
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FLEXIBLE BRAKE HOSEFlexible brake hoses are used
on each front wheel to allow for steering and
suspension movement and at the rear to allow for
rear suspension travel.
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These rubber high-strength hoses can crack,
blister, or leak, and should be inspected at
least every six months.
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Flexible brake hose is made from synthetic yarn
(poly vinyl alcohol, abbreviated PVA) that is
braided into position from multi-end yarn
spindles. By braiding the yarn, all of the
strands operate in tension and, therefore, have
great strength to withstand braking system
pressure over 1,000 psi (6,900 kPa).
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A typical brake hose has an inner tube for
conveying the brake fluid and a cushion liner
that is between the braided layers to prevent the
braids from chafing. An outside jacket is made
from rubber and protects the reinforcement fabric
from moisture and abrasion.The outside covering
is also ribbed as part of the manufacturing
process to hide surface blemishes.
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A constricted brake hose can cause the brakes to
remain applied, thereby causing excessive brake
pad wear and unequal braking. A constricted
flexible brake line can also cause the vehicle to
pull to one side. The hoses should also be
inspected for external wear and damage.
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CAUTION Never allow a disc brake caliper to
hang by the flexible brake line. Damage to the
line can result. Always use a wire to support
the weight of the caliper.
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SUMMARY1. Most brake fluid is amber in color
and polyglycol based.2. DOT 3 is the type of
brake fluid most often recommended for use.3.
Brake fluid absorbs moisture over time and can
remove paint.4. DOT 4 and DOT 5.1 are also
polyglycol-based fluids, whereas DOT 5 is
silicone based.5. Brake fluid should be checked
for proper level and color and tested for
condition or for contamination.6. Brake fluid
should be disposed of according to local and
state guidelines.7. Most brake systems use EPR-
or SBR-type rubber that is compatible with brake
fluid but they can swell in size if exposed to
mineral oil such as power steering fluid, engine
oil, or automatic transmission fluid.8. Brake
lines are double-walled steel with a rust
prevention coating with either a double lap flare
or an ISO flare end.9. Flexible brake hose is
constructed of braided synthetic yarn and EPDM
rubber.
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REVIEW QUESTIONS1. What is the difference
between DOT 3, DOT 4, DOT 5.1, and DOT 5 brake
fluid?2. List four things that should be done
during a thorough inspection of the brake
fluid.3. List four precautions that should be
followed when handling brake fluid.4. What are
the two types of flares used on brake lines?5.
Why are some brake lines coiled?