BRAKE SYSTEMS
T
he International Automotive Technicians’
Network (iATN) is an
organization of about
23,000 professional
technicians, worldwide.
One of its principal functions is to
provide mutual assistance and problem-solving advice for its members
through Internet e-mail forums. One
of the most popular topics—right up
there with driveability and OBD II diagnosis—is brake problems. It may
seem strange that technology as wellestablished as hydraulic brake systems
could generate so many service
headaches for technicians and shop
owners, but it does. And often the solution to a nightmare problem is
found in the application of basic brake
principles. So let’s review “Brake Systems 1A” with an eye toward gaining
some troubleshooting insight.
Brake operation is not abstract theory but a great example of applied
science. We can look at brake trou-
bleshooting from two engineering
viewpoints—conversion of kinetic energy to thermal energy, and the application of hydraulic principles. Let’s
start with stopping power and then
move to the hydraulic applications.
Energy Conversion
and Friction
The power to stop a vehicle comes
from converting kinetic energy (motion) to thermal energy (heat). Kinetic
and thermal energy are two sides of
BY KEN LAYNE
September 1999
71
Maintaining the originally designed stopping power of any vehicle means maintaining the proper heat transfer and friction coefficient of the OE brake system.
the same coin, which is heat transfer.
The primary job of a brake system is to
dissipate heat. In our rush to troubleshoot today’s sophisticated braking
systems, however, we often jump past
basic heat transfer to the high-tech
realm of electronic controllers and
computer programs. In reality, the major factor that determines good or poor
braking performance is simple friction.
This is not going to be a review of a
physics textbook; but if you understand
the simple science at work in a brake
system, you’ll find it easier to keep
your eye on the goal of all brake service—to maintain the same coefficient
of friction and braking force at all four
wheels that has been designed into the
vehicle.
Engineers measure friction by its
coefficient, which is calculated by dividing the force required to slide an
object over a surface by the weight of
the object. For example, if it takes 100
pounds of force to slide a 100-pound
block of iron over a concrete floor, the
coefficient of friction between the two
materials is 1.0. If it takes only 2
pounds of force to slide a 100-pound
block of ice over the same floor, the coefficient is only .02.
Friction exists at two points for
each wheel during braking—between
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September 1999
pad or shoe linings and rotors or
drums, and between the tires and the
road. These are the areas you want to
think about when troubleshooting a
braking problem.
More Is Not Always Better
It’s not a simple proposition that the
highest coefficient of friction will produce the greatest stopping power. In
fact, the coefficient of friction between
brake linings and rotors or drums is always less than 1.0. A coefficient of friction greater than 1.0 means that material actually is being transferred from
one surface to another. Although brake
friction surfaces wear, material is not
transferred from pads to rotors or from
shoe linings to drums. Additionally, excessive friction leads to brake grabbing
and lockup, which, as you know, reduces braking effectiveness.
The coefficient of friction between a
tire and the road can exceed 1.0, however. When this happens, material
transfers from the tire to the road—a
skid mark. That means that the coefficient of friction momentarily exceeded
1.0. These two examples illustrate that
more friction does not always mean better braking. When the coefficient of
friction equals or exceeds 1.0—either at
the brake or at the tire—you get lockup.
And that means you’ve just lost effective
heat transfer and braking power.
Engineers determine the required
coefficient of friction for the best braking performance on a particular vehicle. The friction materials selected for
brake pads and rotors or brake shoes
and drums are designed to give the
best cold-temperature and hot-temperature performance. The best materials
have a coefficient that stays within narrow limits over a wide range of temperatures. If the selected material increases or decreases its coefficient of friction
as temperature changes, the brakes can
fade or grab. This illustrates an important reason to use replacement brake
parts that are equivalent to OE specifications: so braking performance will be
up to the system design requirements.
Three factors affect the coefficient
of friction in a brake system, and these
involve some important service operations on your part:
•The surface finish of both friction
surfaces.
•Temperature.
•The material—the metal of the rotors and drums and the friction material of the pad and shoe linings.
Finish In Style
Carmakers almost universally recommend against machining the surfaces
of brand-new rotors and drums or refinishing rotors unless they’re worn or
scored beyond certain limits. All carmakers do, however, specify definite
surface finish requirements for rotors
and drums.
Because contact between brake
drums and shoe linings is linear, surface finish for a brake drum is not as
critical as it is for rotors. The finish is
not unimportant, however. That’s why
carmakers and parts suppliers tell you
to make the final refinishing cut on a
drum at a shallow depth and slow feed
rate to ensure a uniform finish, free of
grooves and spirals.
Disc brake pads contact the rotors
in an elliptical motion, and the rotor
surface finish is critical for several
reasons. The finish is usually described as “nondirectional,” which
means it doesn’t grind a circular pat-
tern into the pads. A nondirectional
finish is established on a rotor by
sanding each surface for 60 seconds
with 80- to 150-grit sandpaper while
the rotor turns in a lathe.
After sanding both rotor surfaces,
clean each surface with hot water and
detergent to float out the smallest
particles of iron and abrasive materials. Then dry the rotors thoroughly
with clean paper towels. Maintaining
this nondirectional rotor finish and
starting out with clean surfaces retains the designed coefficient of friction between the rotor and pad, and
is a major factor in minimizing noise.
Groovy
You’ve probably heard different explanations for the grooves and
beveled edges of many disc brake
pads (and for the grooves around the
arc of some drum brake linings).
These grooves and edges minimize
noise and help cool the friction surfaces. By doing the latter, the grooves
and bevels help to maintain the original design coefficient of friction.
Some aftermarket pads and
shoes—usually the cheaper varieties—don’t have the OE component
grooves and bevels. As a result, they
often create noise and brake performance problems. High-quality aftermarket parts, on the other hand, usually follow OE design specs and provide the designed braking performance required for the vehicle.
Beat the Heat
As mentioned, the number one job of
a brake system is to dissipate heat,
and many of your service operations
immediately affect this heat dissipation ability. The refinishing limits cast
into drums and rotors mean that you
can’t remove metal beyond the point
where heat dissipation would be compromised. We also mentioned the
fact that grooves and bevels of OE
pads and shoe linings are there for
both noise suppression and cooling,
and aftermarket parts should have
equivalent provisions.
When you inspect drums and rotors, look for cracks, uneven wear, heat
The letters “FE” and “EE” at the ends of these edge codes on the lining material identify the cold and hot coefficients of friction, respectively.
checking, hard spots and other defects
that could reduce braking performance. When you stop to think about
it, it’s clear that these defects fundamentally reduce the heat dissipation
abilities of the brake friction surfaces.
Material Evidence
The friction material used for pads
and shoes can be identified by the
“Automotive Friction Material Edge
Code” printed on the edge of the lining. The letters and numbers of this
code identify the manufacturer, the
material and—most importantly—the
cold and hot coefficients of friction.
From a service standpoint, the friction codes are the most important. You
can often compare them to a chart in
brake parts catalogs to identify the material and whether it’s recommended
for a specific vehicle. These codes are
not the primary factor for selecting replacement linings, but they do indicate
the coefficient of friction, as follows:
C = not over .15
D = over .15 but not over .25
E = over .25 but not over .35
F = over .35 but not over .45
G = over .45 but not over .55
H = over .55
It’s important to use the recommended friction material when re-
placing brake pads and shoes because
using the incorrect type of material
can affect the stopping ability of the
car. These codes indicate only the coefficient of friction, however; they do
not indicate lining quality or hardness. When troubleshooting a brake
problem, it’s often a good idea to install new brake linings that match the
OE friction specifications. This establishes a baseline of what should be
original brake performance for further diagnosis.
Carmakers also sometimes specify
different friction materials for the inboard and outboard pads. You may
find original installations that have an
organic pad on one side of the caliper
and a semimetallic pad on the other.
It’s also common to see linings with a
lower coefficient of friction on the
rear brakes than on the front to minimize rear brake lockup, particularly
on front-drive cars.
Stand On It
The only contact that a motorist has
with the braking system of a vehicle is
through the brake pedal. Whenever
you service a brake system, the customer wants the pedal to “feel” as positive as it did when the car was new.
The customer also wants the car to
September 1999
73
stop with new-car assurance. So before you return the vehicle, check
pedal feel and stopping ability. Regardless of how well you do all the
tasks of a brake job, you’re likely to
have a comeback if the pedal “doesn’t
feel right” to the customer or if the car
“doesn’t seem to stop like it should.”
Wherever hydraulic systems are
used, we’re reminded that they work
because fluids cannot be compressed
as gases can. This applies to transmission, brake, power steering and even
fuel injection systems. In a brake system, the tubing and hoses are “flexible
rods” that transfer pedal force to the
pistons of calipers and wheel cylinders.
Along the way, the hydraulic system
multiplies pedal force through mechanical leverage and through size differences in the pistons of master cylinders, calipers and wheel cylinders.
One of the most important brake
services that you perform is to get
that compressible gas—air—out of
the hydraulic system. Brake bleeding
used to be considered a pretty simple
service. Maybe it was...when cars had
drum brakes at all four wheels and
single-chamber master cylinders. Today, however, bleeding problems are
one of the leading causes of pleas for
help on iATN.
Brake Bleeding Sequences
Bleeding may be performed on all or
just part of the hydraulic system. A
master cylinder usually is bled on the
bench before it’s installed on the car.
Then, final bleeding can be done at
the master cylinder fittings or bleeder
screws to remove any air trapped in
the connections.
Depending on where the hydraulic
system was opened to air, bleeding may
be needed at all four wheels and the
master cylinder; or it might be needed
only at two wheels. If all fittings are disconnected from a dual-reservoir master
cylinder, or if air got into the system
through low fluid level in both cylinder
reservoirs, bleeding is required at the
master cylinder and all four wheels.
If the tubes for one set of wheels
are disconnected at the master cylinder, or if air enters through a low flu-
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September 1999
id level in only the reservoir for one
pair of wheels, only those wheels and
lines need be bled. If there’s any
doubt about air in the system, however, bleed the entire system.
Whether you’re bleeding the complete system or just part of it, begin by
filling the master cylinder with fresh
fluid. If fluid in the reservoir falls below
the level of the master cylinder ports,
air will enter the system. If this happens, bleeding steps must be repeated
to ensure that all air is removed.
The general practice with any type
of bleeding operation is to slip a hose
over the end of the bleeder screw and
place its free end into a jar half filled
with brake fluid. Keeping the end of
the hose submerged in brake fluid
prevents air from being drawn back
into the system. This also lets you see
when air bubbles stop flowing from
the hose, indicating that air has been
removed from that part of the system.
All carmakers recommend a specific
sequence for bleeding the brakes at
the wheels. These recommendations
are included among the brake specifications in a good manual. Before
bleeding the brakes on any vehicle, refer to these recommendations and follow them during the bleeding procedure. If the manufacturer’s recommendations are not available, the following
sequence will work on most vehicles:
1. Master cylinder
2. Combination valve or proportioning valve (if fitted with bleeder screws)
3. Right rear
4. Left rear
5. Right front
6. Left front
This sequence is based on the principles of starting at the highest point
in the system and working downward,
then starting at the wheel farthest
from the master cylinder and working
toward the closest. Some general
rules also are worth remembering:
If the brake system is split between
the front and rear wheels, the rear
wheels (which are farthest from the
master cylinder) usually are bled first.
If the brake system is split diagonally,
the most common sequence is RR-LFLR-RF. This sequence also applies to
most systems with a quick-takeup master cylinder. If you bleed a quick-takeup system in any other sequence, you
may chase air throughout the system.
Exceptions to these general rules
exist, however. Chrysler, for example,
recommends bleeding both rear
brakes before the front brakes, regardless of how the hydraulic system
is split. Some older Japanese imports
have rear drum brakes with a bleeder
screw on only one wheel cylinder. The
manufacturers felt that one bleeder
was satisfactory to remove air from
both rear brakes, but bleeding may
take longer with such an installation.
Remember to press or pull the stem
of a metering valve to lock it open
when using a pressure bleeder. Many
valves require a special clip to hold the
metering valve stem open while you
bleed the front brakes. Also, the pistons of many pressure differential
valves must be recentered after the
system is bled to ensure that the instrument panel warning lamp stays off.
We don’t have space here to go into
the details of various bleeding methods, but you can find the procedures
in a good manual. Remember, vehicles with antilock brakes often require
special bleeding procedures or additional steps.
If you remember the fundamentals
of braking systems and think about
maintaining the designed friction factors and getting the air out of the hydraulic system, your brake service
will be more troublefree with fewer
comebacks.
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Answers to Previous
Assessment Quiz
(Ignition Systems,
July 1999)
1-B 2-C 3-C 4-B 5-A
6-D 7-D 8-C 9-A 10-C