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 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
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 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.
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 BrakesThe 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: 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.