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
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 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.
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
What is Pascal's Law ? (choose 1)
a) For every action there is an equal and opposite reaction.
b) Pressure is distributed equally throughout the system.
c) A body at rest tends to stay at rest until acted on by a force.
d) A person is promoted until they reach their point of incompetence.
A liquid can only be compressed at high temperatures.
a) TRUE b) FALSE
A gas is compressible.
a) TRUE b) FALSE
Terminology
Force happens ___________ the hydraulic system. (choose 1)
a) Inside b) Outside
Force is measured in ______________. (choose 1)
a) Kilopascals b) Newton - meters c) PSI d) pounds
Pressure happens ____________the hydraulic system. (choose 1)
a) Inside b) Outside
Pressure is measured in _____________. (choose 1)
a) Pound - feet b) Newton - meters c) PSI d) pounds
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
How much pressure if piston "C" was .5 sq. in. ? (choose 1)
a) 200 PSI b) 50 PSI d) 50 lbs d) 800 PSI
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.
What does the Master cylinder do? (choose 1)
a) converts the pressure in the brake pedal to force. b) applies the "E" brake
c) converts the force of the brake pedal into pressure. d) works with disc brakes
The lip of the seal always point ______________ the fluid it is sealing. (choose 1)
a) away from b) upward from c) into d) outwards from
In a master cylinder for a drum brake system, why is residual pressure left in the brake lines, even
when the brakes are not applied ? (choose 1)
a) so brakes are always ready to use b) to keep air out
c) to prevent brake fade d) so the brakes are self energizing
Why should there NOT be residual pressure left in the lines of a disc brake system ?
(choose 1)
a) the brakes would drag b) the brakes would not work
c) air would get in the hydraulics d) you would get lousy gas mileage
Why have tandem, or dual master cylinders been used in all cars built since 1966 ? (choose 1)
a) for better gas mileage b) it's cheaper to build
c) so if you lose hydraulics in one circuit, you still have some brakes.
d) so they don't lock up
Why do most front wheel drive cars use dual diagonal brake systems ? (choose 1)
a) because the front brakes on front wheel drive cars are so poor.
b) because the rear brakes on front wheel drive cars are so poor.
c) because it's cheaper d) because it gets better gas mileage
What happens if air gets in the brake lines ? (choose 1)
a) the pedal will be low b) you will have no brakes
c) you will have a spongy pedal d) the brakes will drag
What do wheel cylinders do ? (choose 1)
a) convert the pressure in the hydraulic system to force to push the shoes into the drums.
b) convert the force in the hydraulic system into pressure to push the shoes into the drums.
c) convert the pressure in the hydraulic system to force to push the pads into the rotors.
b) convert the force in the hydraulic system into pressure to push the pads into the rotors.
Why should only clean brake fluid from a closed container be used ? (choose 1)
a) because fluid goes sour after a while b) because fluid absorbs water right out of the air
c) the fluid needs to be fresh d) because fluid absorbs air
What two things will water saturated brake fluid do to a hydraulic system ? (choose 2)
a) rusts the hydraulic parts b) causes brake fade
c) rots the rubber parts d) reduces the boiling temperature of the fluid
What causes brake fade ? (choose 1)
a) overheating b) leaking wheel cylinders
c) shoes out of adjustment d) water on the brake linings
What type of brakes is less prone to fade ? (choose 1)
a) drum brakes b) air brakes c) disc brakes d) parking brakes
Why are asbestos pads being replaced by metallic, and semi-metallic pads ? (choose 1)
a) metallic pads don't get hot b) metallic pads work better
c) metallic pads are cheaper d) asbestos dust is dangerous
What do metallic pads do to the rotors, that asbestos pads didn't ? (choose 1)
a) wear them out faster b) get them hot
c) make them stop d) slow them down
When replacing brake shoes, what important rule should you remember until you know brake
systems really well ? (choose 1)
a) get a friend to help you b) pay a mechanic to do it
c) do one side at a time d) remember how it goes together
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
Why should grease not be used to lubricate the caliper sliders? (choose 1)
a) because grease gets thick in cold conditions
b) because grease gets thin when hot, runs out, and the caliper has no lubrication
c) because grease is messy d) because anti seize is more aesthetically pleasing
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
What should the caliper and holder sliding surfaces be coated with on assembly ? (choose 1)
a) anti-seize b) disc brake quiet c) grease d) loctite
If the wheel doesn't rotate freely after replacing the brake pads, what part needs to be replaced or
rebuilt ? (choose 1)
a) the rotor b) the master cylinder c) the wheel cylinder d) the caliper
The brake pedal is spongy. How would you fix it ? (choose 1)
a) adjust the brakes b) bleed the brakes
c) replace the pads d) rebuild the master cylinder
The brake pedal is low, but hard. How do you fix it ? (choose 1)
a) adjust the brakes, and free up the self adjuster b) bleed the brakes
c) replace the pads d) rebuild the master cylinder
The brake pedal goes to the floor when you step on it. There are no brakes, and there are no leaks.
What is wrong ? (choose 1)
a) the pads are shot b) the wheel cylinders are shot
c) the secondary cup is leaking d) the primary cup is shot
There is a loss of brake fluid, but there are no visible leaks. The car has power brakes. What is
wrong ? (choose 1)
a) the power booster is defective b) the secondary cup is leaking
c) the primary cup is leaking d) the dust boot is defective
The car has a violent pull to the left when the brakes are applied. What is wrong ? (choose 1)
a) the right front brake doesn't work b) the left front brake doesn't work
c) there is too much caster on the left front wheel
d) the left front caliper is dragging
There is a grinding noise when the brakes are applied. What is wrong ? (choose 2)
a) the pads are worn out b) the shoes are worn out
c) the rotors are worn out d) the drums are worn out
The car shakes when the brakes are applied. How would you fix it ? (choose 1)
a) turn the rotors, even if they are too thin b) replace the rotors
c) replace the pads d) replace the calipers
The inner pad is worn, and the outer pad is just about brand new. How would you fix it ? (choose 1)
a) replace the pads, the inner one was defective b) turn the rotors
c) free up the guide pins d) free up the caliper sliders
The brakes squeal. What would NOT be a cause ? (choose 1)
a) the brakes are worn b) the brake squealers are out of adjustment
c) the pads are vibrating d) there is a buildup of brake dust
Name the parts :
Anti - Lock brakes
Which statement is true: (choose 1)
a) anti lock brakes reduce stopping distance
b) anti lock brakes keep the wheel from skidding under all conditions
c) anti lock brakes give you control while you are braking
d) anti lock brakes allow you to follow the car ahead more closely
A reluctor on the rotor, or drum, rotates past the speed sensors creating:(choose 1)
a) a voltage b) a pulse c) resistance d) a memory
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
