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Brake
System
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 is 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 460º 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 very 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.
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 only
used 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. |
|
| 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.
|
| 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: Single
piston floating calipers and four piston 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. This type of 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.
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.
| 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 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.
|
| 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 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 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 of 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. 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 light an ABS warning light to let you know that there is a problem.
If there is a problem, and the anti-lock system will not function but, the brakes will
otherwise function normally. |
Check out this link !
http://www.autoshop-online.com/auto101/brake.html
... and this one ...
http://www.autoshop-online.com/auto101/brake1.html
... and this one ...
http://www.autoshop-online.com/auto101/abs.html
The brake test will be made up from some or all of these questions:
There are sometimes more than one answer that could possibly be right. Always choose the
best one.
Hydraulic Principles
A liquid can only be compressed at high temperatures.
a) TRUE
b)
FALSE
Terminology
Force happens ___________ the hydraulic system. (choose 1)
a) Inside
b)
Outside
Pressure happens ____________the hydraulic system. (choose 1)
a) Inside
b)
Outside
Hydraulic Jack
If a force of 100 lbs was exerted at the end of the handle, how much
pressure would be developed if the area of piston "C" was 1 square inch ?
(choose 1)
a) 100 PSI b)
400 lbs c)
400 PSI d) 100 lbs
If 600 PSI was developed in the system, and cylinder "D" was 10
sq.in. in area, how much force would be developed at the end of the ram ?
(choose 1)
a) 6000 lbs b) 6000 PSI
c) 60 lbs
d) 600 PSI
The mechanical advantage of the handle in the above jack is :
(choose 1)
a) 4 :1
b) 3
:1
c)
6 : 1
d) 10 :1
Brakes
A brake system converts the a) Potential
b) Kinetic
c) Heat d) Radial
energy of the car rolling down the road, to _______________ , by the
________________ of the shoes against the drums, or the pads against the rotors.
When replacing pads, what tool should you use to push the piston back
into the caliper so the new pads will fit ? (choose 1)
a) sledge hammer
b)
16 oz. ball peen
c)
vice grips
d) c - clamp
What chemical should be put on the guide pin threads, to make sure the
pin doesn't come out ? (choose 1)
a) anti-seize b)
disc brake quiet
c)
grease d)
loctite
What should be done to the sliding surfaces of the caliper and holder, to
make sure the caliper slides ? (choose 1)
a) nothing b)
cleaned up with sandpaper
c)
honed out d)
filed
Name the parts :
Anti - Lock brakes
The ABS computer then reads this, and compares it to the vehicle speed.
If it finds a speed sensor reading zero, or approaching zero, it tells the hydraulic
modulator to:(choose 1)
a) allow the brakes to slip.
b)
lock them up
c) take off the brakes
d) pulse the brakes
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