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The
modern automotive brake system has been refined for
over 100 years and has become extremely dependable
and efficient.
The typical brake system consists of disk brakes in
front and either disk or drum brakes in the rear
connected by a system of tubes and hoses that link
the brake at each wheel to the master cylinder.
Other systems that are connected with the brake
system include the parking brakes, power brake
booster and the anti-lock system.
When you step on the brake pedal, you are actually
pushing against a plunger in the master cylinder,
which forces hydraulic oil (brake fluid) through a
series of tubes and hoses to the braking unit at
each wheel. Since hydraulic fluid (or any fluid for
that matter) cannot be compressed, pushing fluid
through a pipe is just like pushing a steel bar
through a pipe. Unlike a steel bar, however, fluid
can be directed through many twists and turns on its
way to its destination, arriving with the exact same
motion and pressure that it started with. It is very
important that the fluid is pure liquid and that
there are no air bubbles in it. Air can compress,
which causes a sponginess to the pedal and severely
reduced braking efficiency. If air is suspected,
then the system must be bled to remove the air.
There are "bleeder screws" at each wheel cylinder
and caliper for this purpose.
 On
a disk brake, the fluid from the master cylinder is
forced into a caliper where it presses against a
piston. The piston, in-turn, squeezes two brake pads
against the disk (rotor), which is attached to the
wheel, forcing it to slow down or stop.
This process is similar to a bicycle brake where two
rubber pads rub against the wheel rim creating
friction.
With drum brakes, fluid is forced into the wheel
cylinder, which pushes the
brake shoes out so that the friction linings are
pressed against the drum, which is attached to the
wheel, causing the wheel to stop.
In either case, the friction surfaces of the pads on
a disk brake system, or the shoes on a drum brake
convert the forward motion of the vehicle into heat.
Heat is what causes the friction surfaces (linings)
of the pads and shoes to eventually wear out and
require replacement.
Let's take a closer look at each of the components
in a brake system and see where other problems can
occur...
Master Cylinder
The master cylinder is located in the engine
compartment on the firewall, directly in front of
the driver's seat. A typical master cylinder is
actually two completely separate master cylinders in
one housing, each handling two wheels. This way if
one side fails, you will still be able to stop the
car. The brake warning light on the dash will light
if either side fails, alerting you to the problem.
Master cylinders have become very reliable and
rarely malfunction; however, the most common problem
that they experience is an internal leak. This will
cause the brake pedal to slowly sink to the floor
when your foot applies steady pressure. Letting go
of the pedal and immediately stepping on it again
brings the pedal back to normal height.
Brake Fluid
Brake fluid is a special oil that has specific
properties. It is designed to withstand cold
temperatures without thickening as well as very high
temperatures without boiling. (If the brake fluid
should boil, it will cause you to have a spongy
pedal and the car will be hard to stop.) Brake fluid
must meet standards that are set by the Department
of Transportation (DOT). The current standard is
DOT-3, which has a boiling point of 460� F. But
check your owners manual to see what your vehicle
manufacturer recommends.
The brake fluid reservoir is on top of the master
cylinder. Most cars today have a transparent
reservoir so that you can see the level without
opening the cover. The brake fluid level will drop
slightly as the brake pads wear. This is a normal
condition and no cause for concern. If the level
drops noticeably over a short period of time or goes
down to about two thirds full, have your brakes
checked as soon as possible. Keep the reservoir
covered except for the amount of time you need to
fill it and never leave a can of brake fluid
uncovered. Brake fluid must maintain a high boiling
point. Exposure to air will cause the fluid to
absorb moisture, which will lower that boiling
point.
NEVER PUT ANYTHING BUT
APPROVED BRAKE FLUID IN YOUR BRAKES. ANYTHING ELSE
CAN CAUSE SUDDEN BRAKE FAILURE! Any other type of
oil or other fluid will react with the brake fluid
and very quickly destroy the rubber seals in the
brake system causing brake failure.
Brake Lines
The brake fluid travels from the master cylinder to
the wheels through a series of steel tubes and
reinforced rubber hoses. Rubber hoses are used only
in places that require flexibility, such as at the
front wheels, which move up and down as well as
steer. The rest of the system uses non-corrosive
seamless steel tubing with special fittings at all
attachment points. If a steel line requires a
repair, the best procedure is to replace the
complete line. If this is not practical, a line can
be repaired using special splice fittings that are
made for brake system repair. You must never use
brass "compression" fittings or copper tubing to
repair a brake system. They are dangerous and
illegal.
Other Components in
the Hydraulic System
Proportioning valve or Equalizer Valve
These
valves are mounted between the master cylinder and
the rear wheels. They are designed to adjust the
pressure between the front and rear brakes depending
on how hard you are stopping. The shorter you stop,
the more of the vehicle's weight is transferred to
the front wheels, in some cases, causing the rear to
lift and the front to dive. These valves are
designed to direct more pressure to the front and
less pressure to the rear the harder you stop. This
minimizes the chance of premature lockup at the rear
wheels.
Pressure Differential Valve
This valve is usually mounted just below the master
cylinder and is responsible for turning the brake
warning light on when it detects a malfunction. It
measures the pressure from the two sections of the
master cylinder and compares them. Since it is
mounted ahead of the proportioning or equalizer
valve, the two pressures it detects should be equal.
If it detects a difference, it means that there is
probably a brake fluid leak somewhere in the system.
Combination Valve
The
Combination valve is simply a proportioning valve
and a pressure differential valve that is combined
into one unit.
Electronic Brake Force Distribution
Newer
cars use the antilock brake hardware and the onboard
computer to replace these proportioning valve
systems with a system called Electronic Brake force
Distribution (EBD) in order to distribute the exact
amount of pressure at each wheel to insure a
balanced brake system.
Disk Brakes
The disk brake is the best brake we have found
so far. Disk brakes are used to stop everything from
cars to locomotives and jumbo jets. Disk brakes wear
longer, are less affected by water, are self
adjusting, self cleaning, less prone to grabbing or
pulling and stop better than any other system
around. The main components of a disk brake are the
Brake Pads, Rotor, Caliper and Caliper Support.
Brake Pads
There are
two brake pads on each caliper. They are constructed
of a metal "shoe" with the lining riveted or
bonded to it. The pads are mounted in the caliper,
one on each side of the rotor. Brake linings used to
be made primarily of asbestos because of its heat
absorbing properties and quiet operation; however,
due to health risks, asbestos has been outlawed, so
new materials are now being used. Brake pads wear
out with use and must be replaced periodically.
There are many types and qualities of pads
available. The differences have to do with brake
life (how long the new pads will last) and noise
(how quiet they are when you step on the brake).
Harder linings tend to last longer and stop better
under heavy use but they may produce an irritating
squeal when they are applied. Technicians that work
on brakes usually have a favorite pad that gives a
good compromise that their customers can live with.
 Brake
pads should be checked for wear periodically. If the
lining wears down to the metal brake shoe, then you
will have a "Metal-to-Metal" condition where the
shoe rubs directly against the rotor causing severe
damage and loss of braking efficiency. Some brake
pads come with a "brake warning sensor" that will
emit a squealing noise when the pads are worn to a
point where they should be changed. This noise will
usually be heard when your foot is off the brake and
disappear when you step on the brake. If you hear
this noise, have your brakes checked as soon as
possible.
Disk or Rotor
The disk
rotor is made of iron with highly machined surfaces
where the brake pads contact it. Just as the
brake pads wear out over time, the rotor also
undergoes some wear, usually in the form of ridges
and groves where the brake pad rubs against it. This
wear pattern exactly matches the wear pattern of the
pads as they seat themselves to the rotor. When the
pads are replaced, the rotor must be machined smooth
to allow the new pads to have an even contact
surface to work with. Only a small amount of
material can be machined off of a rotor before it
becomes unusable and must be replaced. A minimum
thickness measurement is stamped on every rotor and
the technician doing the brake job will measure the
rotor before and after machining it to make sure it
doesn't go below the legal minimum. If a rotor is
cut below the minimum, it will not be able to handle
the high heat that brakes normally generate. This
will cause the brakes to "fade," greatly reducing
their effectiveness to a point where you may not be
able to stop!
Caliper & Support
There are
two main types of calipers: Floating calipers and
fixed calipers. There are other configurations but
these are the most popular. Calipers must be rebuilt
or replaced if they show signs of leaking brake
fluid.
Single Piston Floating Calipers are the most popular
and also least costly to manufacture and service. A
floating caliper "floats" or moves in a track in its
support so that it can center itself over the rotor.
As you apply brake pressure, the hydraulic fluid
pushes in two directions. It forces the piston
against the inner pad, which in turn pushes against
the rotor. It also pushes the caliper in the
opposite direction against the outer pad, pressing
it against the other side of the rotor. Floating
calipers are also available on some vehicles with
two pistons mounted on the same side. Two piston
floating calipers are found on more expensive cars
and can provide an improved braking "feel".
Four Piston Fixed Calipers are mounted rigidly to
the support and are not allowed to move. Instead,
there are two pistons on each side that press the
pads against the rotor. Four piston calipers have a
better feel and are more efficient, but are more
expensive to produce and cost more to service. This
type of caliper is usually found on more expensive
luxury and high performance cars.
Drum Brakes
So if
disk brakes are so great, how come we still have
cars with drum brakes? The reason is cost. While all
vehicles produced for many years have disk brakes on
the front, drum brakes are cheaper to produce for
the
rear wheels. The main reason is the parking brake
system. On drum brakes, adding a parking brake is
the simple addition of a lever, while on disk
brakes, we need a complete mechanism, in some cases,
a complete mechanical drum brake assembly inside the
disk brake rotor! Parking brakes must be a separate
system that does not use hydraulics. It must be
totally mechanical, but more on parking brakes
later.
Drum brakes consist of a backing plate, brake shoes,
brake drum, wheel cylinder, return springs and an
automatic or self-adjusting system. When you apply
the brakes, brake fluid is forced under pressure
into the wheel cylinder, which in turn pushes the
brake shoes into contact with the machined surface
on the inside of the drum. When the pressure is
released, return springs pull the shoes back to
their rest position. As the brake linings wear, the
shoes must travel a greater distance to reach the
drum. When the distance reaches a certain point, a
self-adjusting mechanism automatically reacts by
adjusting the rest position of the shoes so that
they are closer to the drum.
Brake Shoes
Like the
disk pads, brake shoes consist of a steel shoe with
the friction material or lining riveted or bonded to
it. Also like disk pads, the linings eventually wear
out and must be replaced. If the linings are allowed
to wear through to the bare metal shoe, they will
cause severe damage to the brake drum.
Backing Plate
The
backing plate is what holds everything together. It
attaches to the axle and forms a solid surface for
the wheel cylinder, brake shoes and assorted
hardware. It rarely causes any problems.
Brake Drum
Brake
drums are made of iron and have a machined surface
on the inside where the shoes make contact. Just as
with disk rotors, brake drums will show signs of
wear as the brake linings seat themselves against
the machined surface of the drum. When new shoes are
installed, the brake drum should be machined smooth.
Brake drums have a maximum diameter specification
that is stamped on the outside of the drum. When a
drum is machined, it must never exceed that
measurement. If the surface cannot be machined
within that limit, the drum must be replaced.
Wheel Cylinder
The wheel
cylinder consists of a cylinder that has two
pistons, one on each side. Each piston has a rubber
seal and a shaft that connects the piston with a
brake shoe. When brake pressure is applied, the
pistons are forced out pushing the shoes into
contact with the drum. Wheel cylinders must be
rebuilt or replaced if they show signs of leaking.
Return Springs
Return
springs pull the brake shoes back to their rest
position after the pressure is released from the
wheel cylinder. If the springs are weak and do not
return the shoes all the way, it will cause
premature lining wear because the linings will
remain in contact with the drum. A good technician
will examine the springs during a brake job and
recommend their replacement if they show signs of
fatigue. On certain vehicles, the technician may
recommend replacing them even if they look good as
inexpensive insurance.
Self Adjusting System
The parts
of a self adjusting system should be clean and move
freely to insure that the brakes maintain their
adjustment over the life of the linings. If the self
adjusters stop working, you will notice that you
will have to step down further and further on the
brake pedal before you feel the brakes begin to
engage. Disk brakes are self adjusting by nature and
do not require any type of mechanism. When a
technician performs a brake job, aside from checking
the return springs, he will also clean and lubricate
the self adjusting parts where necessary.
Parking Brakes
The parking brake (a.k.a. emergency brake) system
controls the rear brakes through a series of steel
cables that are connected to either a hand lever or
a foot pedal. The idea is that the system is fully
mechanical and completely bypasses the hydraulic
system so that the vehicle can be brought to a stop
even if there is a total brake failure.
On drum
brakes, the cable pulls on a lever mounted in the
rear brake and is directly connected to the brake
shoes. this has the effect of bypassing the wheel
cylinder and controlling the brakes directly.
Disk brakes on the rear wheels add additional
complication for parking brake systems. There are
two main designs for adding a mechanical parking
brake to rear disk brakes. The first type uses the
existing rear wheel caliper and adds a lever
attached to a mechanical corkscrew device inside the
caliper piston. When the parking brake cable pulls
on the lever, this corkscrew device pushes the
piston against the pads, thereby bypassing the
hydraulic system, to stop the vehicle. This type of
system is primarily used with single piston floating
calipers, if the caliper is of the four piston fixed
type, then that type of system can't be used. The
other system uses a complete mechanical drum brake
unit mounted inside the rear rotor. The brake shoes
on this system are connected to a lever that is
pulled by the parking brake cable to activate the
brakes. The brake "drum" is actually the inside part
of the rear brake rotor.
On cars with automatic transmissions, the parking
brake is rarely used. This can cause a couple of
problems. The biggest problem is that the brake
cables tend to get corroded and eventually seize up
causing the parking brake to become inoperative. By
using the parking brake from time to time, the
cables stay clean and functional. Another problem
comes from the fact that the self adjusting
mechanism on certain brake systems uses the parking
brake actuation to adjust the brakes. If the parking
brake is never used, then the brakes never get
adjusted.
Power Brake Booster
The power
brake booster is mounted on the firewall directly
behind the master cylinder and, along with the
master cylinder, is directly connected with the
brake pedal. Its purpose is to amplify the available
foot pressure applied to the brake pedal so that the
amount of foot pressure required to stop even the
largest vehicle is minimal. Power for the booster
comes from engine vacuum. The automobile engine
produces vacuum as a by-product of normal operation
and is freely available for use in powering
accessories such as the power brake booster. Vacuum
enters the booster through a check valve on the
booster. The check valve is connected to the engine
with a rubber hose and acts as a one-way valve that
allows vacuum to enter the booster but does not let
it escape. The booster is an empty shell that is
divided into two chambers by a rubber diaphragm.
There is a valve in the diaphragm that remains open
while your foot is off the brake pedal so that
vacuum is allowed to fill both chambers. When you
step on the brake pedal, the valve in the diaphragm
closes, separating the two chambers and another
valve opens to allow air in the chamber on the brake
pedal side. This is what provides the power assist.
Power boosters are very reliable and cause few
problems of their own, however, other things can
contribute to a loss of power assist. In order to
have power assist, the engine must be running. If
the engine stalls or shuts off while you are
driving, you will have a small reserve of power
assist for two or three pedal applications but,
after that, the brakes will be extremely hard to
apply and you must put as much pressure as you can
to bring the vehicle to a stop.
Anti-Lock Brakes (ABS)
The most efficient braking pressure takes place just
before each wheel locks up. When you slam on the
brakes in a panic stop and the wheels lock up,
causing a screeching sound and leaving strips of
rubber on the pavement, you do not stop the vehicle
nearly as short as it is capable of stopping. Also,
while the wheels are locked up, you loose all
steering control so that, if you have an opportunity
to steer around the obstacle, you will not be able
to do so. Another problem occurs during an extended
skid is that you will burn a patch of rubber off the
tire, which causes a "flat spot" on the tread that
will produce an annoying thumping sound as you
drive.
Anti-lock brake systems solve this lockup problem by
rapidly pumping the brakes whenever the system
detects a wheel that is locked up. In most cases,
only the wheel that is locked will be pumped, while
full braking pressure stays available to the other
wheels. This effect allows you to stop in the
shortest amount of time while maintaining full
steering control even if one or more wheels are on
ice. The system uses a computer to monitor the speed
of each wheel. When it detects that one or more
wheels have stopped or are turning much slower than
the remaining wheels, the computer sends a signal to
momentarily remove and reapply or pulse the pressure
to the affected wheels to allow them to continue
turning. This "pumping" of the brakes occurs at ten
or more times a second, far faster then a human can
pump the brakes manually. If you step on the brakes
hard enough to engage the anti-lock system, you may
feel a strong vibration in the brake pedal. This is
a normal condition and indicates that the system is
working, however, it can be disconcerting to some
people who don't expect it. If your vehicle has
anti-lock brakes, read your owner's manual to find
out more about it.
The system consists of an electronic control unit, a
hydraulic actuator, and wheel speed sensors at each
wheel. If the control unit detects a malfunction in
the system, it will illuminate an ABS warning light
on the dash to let you know that there is a problem.
If there is a problem, the anti-lock system will not
function but the brakes will otherwise function
normally.
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