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To register your Bendix Performance Replacement Plan
online, click here:
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Anti-Lock Brake System |
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ABS was designed to help you
maintain directional control during emergency stops and when road
conditions are poor. By maintaining control, you have better chances of
avoiding a crash. ABS is especially useful on wet and slippery roads, and
it prevents wheels from locking up.
 Note:
You should never pump the brake pedal on a car with ABS, since the system
itself “pumps” the brakes automatically. Apply firm
and continuous pressure to the brake pedal to activate ABS operation. When
the ABS system operates, you may feel a pulsating sensation from the brake
pedal. When ABS operation is no longer needed, the braking system reverts
to conventional hydraulic operation.
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Today, virtually all cars come with ABS as standard
equipment or as an option. The typical ABS system includes wheel-speed
sensors, a hydraulic control unit, and an electronic control
unit. When you apply the brake pedal, the electronic control unit
monitors and compares the signals from the wheel-speed sensors. If the
electronic control unit senses rapid deceleration (impending lock-up) at a
given wheel, the electronic control unit commands the hydraulic control
unit to reduce hydraulic pressure to that wheel. This type of pressure
limiting is similar to pumping the brake pedal, only much faster. |
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When turning the ignition
switch to the on position, the amber BRAKE, ANTILOCK or ABS light on the
instrument panel should glow momentarily, and then turn off. If the light
stays on or flashes, or comes on while driving, it indicates a fault in
the ABS system, and you should have it checked. |
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Disc Brakes |
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Most modern cars have disc brakes on the front
wheels, and some have disc brakes on all four wheels. Disc
brakes are a tremendous improvement over drums. They dissipate heat
much better, they don't lose their stopping power when wet, their
linings last a long, long time and they stay in adjustment
automatically. |
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The main components of disc brakes are:
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- The brake
pads
- The
calipers, which contains a piston
- The rotors,
which are mounted to the hubs
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Disc brakes are
similar to the brakes on a bicycle. The brake pads squeeze the rotor
instead of the wheel, and the force is transmitted hydraulically
instead of through a cable. Friction between the pads and the
disc slows the disc down to stop the vehicle. |
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 When the brake pedal
is depressed, it pressurizes the master cylinder which forces brake
fluid into the calipers. This forces the caliper pistons outward
and against the brake pads. The brake pads then squeeze the
rotating rotor. The rotor is attached to and stops the wheel.
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When the brake pedal
is released, a small amount of brake fluid returns to the master
cylinder, lowering the hydraulic pressure on the piston. The
clamping action of the brake pads against the rotor is then released. |
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Disc Brake Service
The most common
service required for disc brakes is changing the brake pads. When
enough of the friction material is worn away, the wear indicator will
contact the disc and make a squealing sound. This means it is time for
new brake pads.
Sometimes, deep scores get worn into brake
rotors. This can happen if a worn-out brake pad is left on the car for
too long. Brake rotors can also warp, that is, lose their flatness. If
this happens, the brakes may shudder or vibrate when you stop. Both of
these problems can sometimes be fixed by machining the rotors. Some
material is removed from both sides of the rotors to restore the flat,
smooth surface. Because the process removes material, however, brake rotors get
thinner every time they are machined. All brake rotors have a
specification for the minimum allowable thickness before they need to
be replaced.
Most authorities recommend, and we
agree, that caliper hydraulic service should be part of
a standard disc brake job much more often than it actually does occur
in many repair shops. Why? For one, to insure again piston seizure.
And secondly, when the caliper pistons are forced back into their
bores to accommodate the thick, new pads, the fluid seals have to work
against a surface that's probably corroded and/or contaminated, which
very often will cause a leak. |
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Drum Brakes |
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Drum brakes were the first
type of brakes used on motor vehicles. Many cars still have drum
brakes on the rear wheels and disc brakes on the front.
The main components of
drum brakes include:
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Brake shoes mounted on a stationery
backing plate
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Brake Drums
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Wheel Cylinders
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an adjuster mechanism
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various springs and
hardware
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an emergency brake
mechanism
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When you push on the brake
pedal, the force of your leg generates hydraulic pressure in the master
cylinder, which then flows through the hydraulic lines and hoses to the
wheel cylinders. This pressure moves the wheel cylinder pistons
outward until the brake shoes contact the drum. As the
stationery brake shoes contact the rotating drum, there is a kind of wedging action, which has
the effect of pressing the shoes into the drum with more force.
 Brake
shoes are the parts that support the brake lining. The lining is
either riveted or bonded to the face or table of the shoe. The face is
formed to fit the contour of the drum. The force used to push the brake
shoes against the drum is created by the applied force of the wheel
cylinder and the movement of the shoe within the drum.
When the brake pedal is released, the shoe return
springs pull the brake shoes back to the released position. Other springs
& pins help hold the brake shoes in place.
For the drum brakes to function
correctly, the brake shoes must remain close to the drum without touching
it. Drum brakes have an automatic adjuster. The automatic adjuster
mechanism maintains correct operating clearance between the brake
lining and brake drum by adjusting the brake shoes in direct proportion to
lining wear. |
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Drum Brake Service
Generally, drum brakes are more prone to problems
than discs. The most common service
required for drum brakes is changing the brake shoes.
Brake shoes should be
replaced when the friction material has worn down to within 1/32 inch (0.8
mm) of the rivets. If the friction material is bonded to the backing plate
(no rivets), then the shoes should be replaced when they have only 1/16
inch (1.6 mm) of material left. |
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Just as in disc brakes, deep scores sometimes get
worn into brake drums. If a worn-out brake shoe is used for too long, the
rivets that hold the friction material to the backing can wear grooves
into the drum. A badly scored drum can sometimes be repaired by machining.
Where disc brakes have a minimum allowable thickness, drum brakes have a maximum allowable diameter. Since the contact surface is the inside
of the drum, as you remove material from the drum brake the diameter gets
bigger.
Wheel cylinders are apt to develop leaks and
they are particularly prone to corrosion and
seepage, which can result in reduced or nonexistent stopping performance,
and contamination of the shoes. |
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Striking a Balance
Most cars that use front disc brakes and rear drum
brakes use a metering and proportioning valve in the system
to achieve balanced braking between the front and rear wheels. The purpose
of the metering valve is to improve braking balance, particularly during
light brake application. The metering valve keeps the front discs
from operating until the rear drums have started to work. The
proportioning valve reduces the hydraulic pressure at the rear drum
brakes when high pressure is required at the front disc brakes. |
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Emergency Brakes |
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The parking brake assembly is designed to apply the
brakes mechanically to prevent the car from rolling when parked, or to
stop the car in the event of a complete hydraulic failure. Most parking
brakes operate on the two rear brakes. |
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On a typical drum brake
system, the emergency brake cable moves a parking brake lever attached to
the secondary brake shoe. When
the parking brake is applied, the cable pulls the lower end of the parking
brake lever forward, causing the connecting strut to push the primary
brake shoe forward. At the same time, the upper end of the lever pushes
the secondary brake shoe rearward. The combined action of the lever and
strut expands the brake shoes, forcing them against the drum to develop
brake action. |
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In cars with disc brakes
on all four wheels, an emergency brake has to be actuated by a separate
mechanism than the primary brakes in case of a total primary brake
failure. Most cars use a cable to actuate the emergency brake. |
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Some cars with four-wheel disc brakes have a
separate drum brake integrated into the hub of the rear wheels. This drum
brake is only for the emergency brake system, and it is actuated only by
the cable; it has no hydraulics. Other cars have a lever that turns a
screw, or actuates a cam, which presses the piston of the disc brake.
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Have
your car’s brakes inspected annually to make sure everything’s OKAY. It’s
always best to be able to plan ahead for brake work by knowing brake
condition as your car ages. Planning can also save you money, because the
brakes won’t get to the “metal-to-metal” point, which usually means
expensive rotor or drum replacement.
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So What Is a "Complete" Brake Job?
Unless you're a Tibetan cave
dweller, you've seen the ads by mass merchandisers, tire stores, and
muffler chains: "Complete Brake Service $39.95." Sure, they hang fresh
linings and maybe even do a little machining, but the word "complete" is a
serious exaggeration.
Typically, what a low-dollar,
loss-leader job includes is new linings, disc/drum machining, and topping
off the reservoir -- not the kind package we feel safe about letting out
of our shop. The following, in our opinion, comprises a complete brake
job:
- Road test
- Replace pads and shoes
- Machine drums and rotors;
replace if necessary
- Inspect caliper bushings,
guide pins, pad shims, etc.; replace if necessary; overhaul caliper
cylinders, lube slides
- Clean, repack, & adjust wheel
bearings
- Replace grease seals
- Replace drum brake hardware
- Clean & lube star wheel
adjusters
- Smooth out & lube drum backing
plate shoe pads.
- Inspect wheel cylinders;
replace if necessary
- Check master cylinder; replace
if necessary
- Inspect hydraulic hoses and
lines; replace if necessary
- If there's a braking
performance complaint, check the metering/proportioning/combination
valve
- Flush, refill hydraulic system
(includes bleeding)
- Apply anti-seize to bleeder
screw threads
- Inspect/lube or replace
parking brake cables, then adjust lever travel
- Check booster operation
- Check vacuum booster hose
- Road test again
And just what do we charge for all
of that? Certainly, it amounts to a great deal more effort than anybody
but a third-world laborer would do for forty bucks. The answer lies in
careful inspection and good judgment. Those two factors will make it
possible to give the car what it really needs. In other words, what's
practical, safe, and reasonably safe from future annoyances. That's
why we have no "set" prices for brake work. But we'll be glad to
look it over for you and give you an honest & legitimate estimate.
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