GLOSSARY OF BASIC BRAKE TERMINOLOGY
Antilock Brake System. Its purpose is to prevent wheel lockup and skidding during panic stops or while braking on slick or wet
surfaces. Antilock braking reduces stopping distances under such circumstances, improves vehicle stability and safety, and
allows the driver to retain a certain amount of steering control in an emergency situation. It plays no role in normal braking.
ABS WARNING LIGHT
An indicator light on the instrument panel that warns the driver when there's a problem with the ABS system. When the ignition
is first switched on, the ABS warning light should come on and remain on for several seconds for a bulb check. If the light fails
to go out or comes on while driving, it signals a potential problem with the ABS system. The ABS system is usually disabled if
the ABS warning light is on while driving (this should have no effect on normal braking -- unless the brake warning light is also
on). The light is also used for diagnostic purposes when retrieving flash codes (trouble codes) from the ABS module.
A pressure storage chamber or reservoir that's used for power assist braking as well as antilock braking on integral ABS
systems, and for temporarily holding brake fluid on some nonintegral ABS systems during antilock braking.
The accumulator on integral ABS systems consists of a nitrogen charged chamber sealed by a rubber diaphragm. As brake
fluid is pumped into the accumulator by the ABS pump, it compresses the nitrogen gas and stores up pressure. The stored
pressure (up to 2700 psi) is then used for power assist braking and for reapplying the brakes during the hold-release-reapply
cycle during antilock braking.
SERVICE NOTE: The accumulator must be depressurized prior to doing any type of brake service work. This is done by
pumping the brake pedal 40 times while the ignition key is off.
In nonintegral ABS systems where an accumulator is used to temporarily hold fluid during the release phase of the
hold-release-reapply ABS cycle, the accumulator consists of a spring loaded diaphragm. This type of accumulator does not
have to be depressurized prior to performing brake service.
Brake hardware that attaches to the pads on disc brakes to help prevent pad vibrations that cause noise.
A nonflammable, heat resistant mineral fiber that's used in brake linings. Because of health concerns over the inhalation of
asbestos dust (which is carcinogenic), nonasbestos brake linings are now used as original equipment on most new vehicles
today. Asbestos linings are still available in the aftermarket for replacement applications, but most brake lining manufacturers
recommend substituting nonasbestos organic (NAO) linings for asbestos when the brakes are relined. Asbestos linings should
never be used in applications are require semi-metallic linings.
Automatic Slip Regulation. See Traction Control.
The stamped steel plate upon which the brake shoes, wheel cylinder and anchor pin are mounted for a drum brake. Raised
pads or bosses on the backing plate help support the shoes. If the pads have become worn, the backing plate should be
A wear condition in a brake drum where the drum has weakened and expanded outward at the open end like a bell. This
prevents parallel contact with the brake shoes. The drum must be replaced.
A procedure for purging air from a master cylinder before it is installed on the vehicle.
A piece of rubber hose or clear plastic tubing used when bleeding the brakes. One end of the hose is pushed over the bleeder
screw and the other is submerged in a container or jar partially filled with brake fluid.
A small screw valve on the wheel cylinder or caliper that allows brake fluid and air to be bled from the component. Bleeder
screws are also used on some master cylinders and ABS modulators.
BLEEDING THE BRAKES
A procedure for purging air and/or old brake fluid from the lines and other hydraulic components in the brake system. Bleeding
is necessary anytime the hydraulic system has been opened up for repairs, and is recommended as a preventative
maintenance procedure when the brakes are relined and/or every two years to remove moisture contamination in the fluid.
Brake linings that are attached to the shoes or pads with an adhesive rather than rivets.
A proportioning valve is used to reduce the amount of brake pressure to the rear brakes to compensate for the forward weight
transfer that occurs during braking as well as the normal front/rear weight distribution on the suspension. This is necessary to
prevent rear wheel lockup and skidding.
BRAKE CLEANING CHEMICALS
Specially formulated chemical solvents that are used to remove brake dust, dirt and grease from brake components during
routine service work. Some are aerosols while others are brushed on. The effectiveness of the product as well as its
flammability and toxicity depend upon the ingredients in it. Ordinary petroleum based solvents or degreasers should never be
used for cleaning brake parts because petroleum solvents leave behind a residue that can damage rubber parts.
A loss of braking ability caused by excessive heat. Repeated brake applications or continuous braking (as when driving down a
mountain) can overheat the brake linings and cause them to lose friction. The driver has to press harder and harder on the
pedal to slow the vehicle, and eventually reaches the point where the brakes cease to slow the vehicle at all. The condition is
usually accompanied by a burning smell and/or smoke from the brakes. It can be prevented by using low gear when driving
down a mountain (using the engine's compression to help brake the vehicle), and by allowing the brakes to cool periodically by
not riding them continuously.
Another type of brake fade can occur with drum brakes if water gets inside the drum (as when driving through puddles). The
water acts like a lubricant and prevents the shoes from exerting their normal friction.
The special hydraulic fluid that's used in the brake system to transmit force from the master cylinder to the wheel brakes (see
DOT 3, 4 and 5 fluid)
A special type of high temperature silicone or synthetic grease that can be used to lubricate and protect caliper mounts and
guide pins, drum brake self-adjuster and star wheel mechanisms, the raised pads on the brake backing plates that support the
shoes, and parking brake mechanisms. Ordinary chassis grease should never be used for this purpose because it can't take
the heat. It will run and may contaminate the brake linings.
The flexible rubber hose that connects the steel brake lines to the brake calipers and wheel cylinders. Hoses come with male
and female ends, different lengths and diameters. Hoses harden and crack with age, and must be replaced if cracked,
blistered, torn, leaking or otherwise damaged. The replacement hose must be identical to the original: too long and it may rub,
too short and it may be pulled apart by steering or suspension motions.
A machine for resurfacing brake drums and rotors. There are two basic types: bench lathes for off-car resurfacing of drums
and rotors, and on-car lathes (or grinders) for resurfacing rotors only. On-car lathes can save disassembly time on vehicles
with four-wheel disc brake systems or where rotor removal is difficult.
A special type of high pressure steel tubing that carries the fluid from the master cylinder to the wheel brakes. Use only
"approved" steel tubing for brake lines, never any other type of tubing. Lines that are badly rusted, corroded or damaged
should always be replaced. The end fitting must also be the proper type (double flare or ISO) to mate with the other
components in the system.
BRAKE PEDAL SWITCH
Used on some ABS systems to signal the control module when the brakes are being applied.
The tendency of a vehicle to steer or pull to one side when the brakes are applied. This condition may be caused by uneven
braking side-to-side as a result of a hydraulic failure, a frozen caliper or wheel cylinder, or fluid or grease contaminated brake
linings on one side.
A special curved tool with flat ends for turning the star wheel adjusters on drum brakes. The end of the tool is inserted through
an opening in the brake backing plate or brake drum and used to adjust the clearance between the shoes and drum.
BRAKE WARNING LIGHT
The red brake warning light alerts the driver if (1) the parking brake is on, (2) there is a loss of hydraulic pressure in either of
the vehicles two brake circuits, or (3) if the fluid level is low. A red warning light usually indicates a fluid leak that needs to be
investigated and repaired. It may not be safe to operate a vehicle if the brake warning light is on.
The component in a disc brake that applies the hydraulic force to press the brake pads against the rotor. The caliper may have
one to four pistons, with either a "fixed" or "floating" mount. Caliper problems include leaks caused by seal and bore wear,
frozen pistons caused by corrosion, and uneven pad wear caused by corrosion of the caliper mount or guide pins (floating
calipers only) or frozen pistons. Calipers may be rebuilt or replaced.
A brake valve that combines two or more valves (proportioning valve, pressure differential valve and/or metering valve) in a
A special tool for resurfacing the cylinder bores in wheel cylinders and master cylinders. Master cylinders with aluminum
housings should NOT be resurfaced because honing removes the protective anodized coating inside the bore.
A way of splitting the hydraulic system so the right front and left rear brakes are on one circuit, and the left front and right rear
are on another (as opposed to combining both front brakes on one circuit and both rears on the other). This type of
arrangement is often used on front-wheel drive cars.
A special measuring tool with a gauge indicator that can be used to check rotor runout and wheel bearing play.
A type of brake that uses a flat disc or rotor rather than a drum for the friction surface. Pads are pressed against both sides of
the disc or rotor by a caliper assembly. Disc brakes can usually handle higher temperatures than drums, and will not trap water
or dirt like a drum.
DOT 3 BRAKE FLUID
Brake fluid that meets the Department of Transportation specifications for glycol based fluids with a wet boiling point (lowest
allowable after it's been in use) of 284 degrees F. and a dry boiling point of 401 degrees F. DOT 3 fluid is the type commonly
specified by most vehicle manufacturers. Because it is glycol based, it absorbs moisture over time (hygroscopic). This lowers
its effective boiling point and promotes internal corrosion in the brake system. For this reason, the brake fluid should be
replaced periodically (every two years is recommended by many experts) and every time the brakes are relined or serviced.
DOT 4 BRAKE FLUID
A "heavy-duty" glycol based brake fluid with a slightly higher wet boiling point of 311 degrees F. and a dry boiling point of 446
degrees. This type of fluid is sometimes specified for performance vehicles or those subject to high brake temperatures.
DOT 5 BRAKE FLUID
A silicone based fluid that does not absorb moisture and has a boiling point of at least 500 degrees F. DOT 5 fluid does not
have to be changed periodically and can minimize brake system corrosion, but is very expensive compared to DOT 3 or DOT 4
fluid (it costs three to five times as much). It is NOT recommended for ABS systems because it can aerate when cycled
through small orifices. It also will not mix with DOT 3 or DOT 4 brake fluid.
DOUBLE FLARE TUBING
The folded over end of flared tubing that's necessary with steel brake lines. The double flare improves strength. Double-flare
and ISO fittings are not interchangeable.
The ring-shaped housing that's provides the friction surface for the internally mounted shoes in a drum brake system. Most
drums are cast iron, but may be aluminum with a cst iron liner.
DUAL MASTER CYLINDER
A master cylinder that is divided into two separate hydraulic circuits. This leaves one circuit to stop the car if the other develops
a leak. Each circuit has its own piston and fluid reservoir.
A brake design that provides servo action regardless of which way the drum is turning (forward or reverse).
A rubber boot on a disc brake caliper that keeps dirt and moisture out from getting between the piston and caliper bore. The
boot (as well as the piston seal) hardens with age, and should be replaced when the brakes are relined.
Letter markings on the edge or side of brake linings that indicates the friction characteristics of the lining material and the
A device in the parking brake system that equalizes the pull on both rear brake cables.
See Flash Code & Trouble Code.
The name given to fault codes or trouble codes that are read by counting flashes of the ABS warning light. Though not
available on some ABS systems, flash codes provide essential diagnostic information for troubleshooting common ABS
problems. To read any codes that may be stored in the ABS control module's memory, the ABS system must first be put into a
special diagnostic mode by grounding a terminal on the vehicle's diagnostic connector. The codes are then flashed out via the
ABS warning light. By carefully counting the flashes and pauses, a numeric code is deciphered that tells the technician which
diagnostic chart to refer to in the service manual. A series of step-by-step checks must then be made to isolate the faulty
A part of the master cylinder assembly that holds fluid for the brake system. On many cars today, the fluid reservoir is made of
clear plastic so the fluid level inside can be inspected without having to open the reservoir. The fluid level should be maintained
at the full mark so air does not enter the system. Use the type of brake fluid specified by the vehicle manufacturer for the
A type of caliper that is rigidly mounted over the rotor, and has either two or four opposing pistons to squeeze the pads against
A type of caliper that slides on guide pins or clips over the rotor so a single piston can squeeze both pads against the rotor.
When the piston moves outward, the inboard pad contacts the rotor first, causing the caliper to slide sideways slightly until the
outboard pad also contacts the rotor. When the piston retracts, the pads are kicked out away from the rotor and the caliper
self-centers over the rotor. Uneven pad wear can be caused if corrosion on the caliper mount or guide pins prevents the caliper
A method of bleeding the brakes that allows fluid to dribble out of the open bleeder screws by gravity. It's a slow process and
rarely used except in applications that require it because of metering valve arrangements that prevent normal manual or power
HOLD DOWN SPRINGS
Hardware in a drum brake system that holds the shoes to the backing plate. The springs can weaken with age, and should be
replaced when the brakes are relined.
A term that refers to the tendency to absorb moisture. Glycol based brake fluid (DOT 3 & 4) are hygroscopic, but silicone fluid
(DOT 5) is not.
A type of power brake system that uses hydraulic pressure from the power steering pump to provide brake assist rather than
An ABS system where the master cylinder is combined with the hydraulic modulator into one assembly. A high pressure pump
and pressure accumulator are used to provide power assist instead of a vacuum booster. An integral ABS system is more
expensive for the vehicle manufacturer to install on a vehicle, and more expensive to service because the master cylinder and
hydraulic modulator must be replaced as an assembly if either is defective.
ISO FLARE TUBING
A cupped type of end flaring that's used on steel brake lines. ISO stands for International Standards Organization. ISO and
double flare tubing are not interchangeable.
The amount of sideways variation in the movement of a brake rotor or wheel. Lateral runout can be checked by positioning a
dial indicator against the rotor and then turning the rotor. The amount of runout can then compared to specs to determine if the
rotor needs to be resurfaced or replaced.
LOAD SENSING PROPORTIONING VALVE
A type of brake proportioning valve that changes the amount of brake force to the rear brakes according to how a vehicle is
loaded. Most use a valve actuated by a lever between the rear suspension and chassis. As the vehicle is loaded, changes in
ride height move the lever and open the valve further to increase rear braking.
A type of replacement caliper that comes complete with pads and hardware (as opposed to a bare replacement caliper where
the pads and other hardware have to be purchased separately).
A procedure for bleeding the brakes using the brake pedal to push fluid through the lines. The bleeder screws are opened one
at a time (in the specified sequence, usually starting with the one furthest away from the master cylinder). A hose is then
attached to the bleeder screw and immersed in a container partially filled with brake fluid. The pedal is then slowly pumped to
push fluid through the line to remove air. The bleeder screw should be closed after every stroke of the pedal to prevent air from
being drawn back into the line (which may require the assistance of a helper). Because of this, most technicians prefer to use a
power bleeder because it's much faster.
The component in the hydraulic brake system that converts the force exerted on the brake pedal by the driver into hydraulic
pressure that applies the brakes.
A valve that limits the pressure supplied to the front disc brakes so the rear drum brakes (which take more force to apply) will
apply simultaneously. Metering valves are used primarily on a few import applications.
The hydraulic control unit that contains the valves and solenoids that regulate the brake circuits during antilock braking. The
modular is part of the master cylinder assembly in "integral" ABS systems, but separate in "nonintegral" ABS systems. As a
rule, the modulator does not contain serviceable components and must be replaced as a unit if there's an internal problem.
The solenoids, pump and accumulator are the only components on the modulator that can be replaced separately.
Abbreviation for "Non Asbestos Organic," a type of brake lining material that does not contain asbestos as a reinforcing fiber.
NAO linings can be substituted for asbestos linings when the brakes are relined.
A type of rotor finish that's applied by sanding rotors after they've been turned. The random direction of the scratches on the
surface help reduce pad vibrations and noise.
An ABS system that uses a conventional master cylinder and vacuum booster with a separate hydraulic modulator assembly.
This type of system is also called an "add-on" ABS system because it is essentially added onto the existing brake system.
Refers to variations in the thickness of the rotor, or the parallel alignment of the two surfaces of the rotor. Parallelism is
checked with a micrometer at six or more different points around the circumference of the rotor. If the thickness varies more
than the specs allow, the rotor must be resurfaced or replaced.
A condition where the brake pedal pulsates or vibrates when the brakes are applied. This is usually due to a warped brake
rotor. The cure is to resurface or replace the rotor. On vehicles with antilock brakes, pedal pulsation and noise during a panic
stop or when braking on a wet or slick surface does not indicate a problem. It results from pressure feedback as the ABS
system cycles pressure in the brake lines.
The element inside a caliper or wheel cylinder that is moved by hydraulic pressure to apply the brakes. Wheel cylinder pistons
are usually steel but caliper pistons may be steel, aluminum or phenolic.
A "plastic" piston made of phenolic resin. Phenolic pistons are lightweight, corrosion proof and don't transfer heat to the brake
fluid like metal pistons.
See Pressure Bleeding.
A procedure for purging the hydraulic system of air and old fluid that uses a pressurized tank to force fluid through the system.
The pressure bleeder is attached to the master cylinder reservoir. When the brake bleeder screws are opened, the fluid is
forced through the lines. Pressure bleeding is a fast, one-person job.
PRESSURE DIFFERENTIAL VALVE
A safety valve that turns on the brake warning light if there's a loss of pressure in either side of the brake system. The valve
remains in a neutral position between the two brake circuits as long as pressure is the same on both sides. A loss of fluid on
either side causes the valve to move when the brakes are applied and illuminate the brake warning light. The valve is
self-centering on most applications, but must be reset on others if a problem has occurred.
PRIMARY BRAKE SHOE
In a self-energizing drum brake, it is the brake shoe that is pulled away from the anchor by the rotation of the drum. It is usually
the forward shoe.
A brake balance valve that reduces pressure to the rear brakes to prevent rear wheel lockup during hard stops.
QUICK TAKE-UP MASTER CYLINDER
A type of master cylinder used with brake calipers that are designed to reduce brake drag for improved fuel economy. The
caliper piston seals retract the pistons when the brakes are released so the pads don't drag against the rotors. Because the
caliper pistons have to travel further when applied, the master cylinder has a stepped bore that delivers a larger volume of fluid
when the brake pedal is initially depressed. There's also a special "quick take-up" valve in the master cylinder that controls the
movement of this extra fluid.
Rear Antilock Brake System. Ford's name for rear wheel ABS. See RWAL and Rear Wheel ABS.
REAR WHEEL ABS
A type of ABS system that only involves the rear wheels. Commonly used on pickup trucks and vans, rear-wheel ABS provides
skid control with varying vehicle loads. This type of ABS system uses only a single speed sensor in the transmission or
differential for both rear wheels. See RABS and RWAL.
The springs between the shoes in a drum brake that retract the shoes away from the drum when the brakes are released. The
springs weaken with age and heat, and should be replaced when the brakes are relined.
The disc that provides the friction surface in a disc brake. Pads rub against both sides of the rotor to generate friction. The
rotor may be solid or vented. The condition of the rotor is very important for proper braking. High spots that result from
warpage can cause pedal pulsation when the brakes are applied.
Rear Wheel Anti-Lock brakes. A term used by General Motors and Chrysler for rear-wheel antilock braking.
A hand-held electronic tool that plugs into a vehicle's diagnostic connector to access fault codes and other diagnostic
information. Scan tools can be used for ABS diagnostics as well as engine and other diagnostics.
In a self-energizing drum brake, it is the shoe that is pushed into contact with the anchor by the rotating drum. It is usually the
See Self-Servo Brakes.
A type of drum brake in which one shoe helps apply the other. This multiplies the braking force and reduces pedal effort.
A type of brake lining material that uses metallic fibers such as steel wool as a primary ingredient. Semi-metallic ("semi-met")
brake linings are excellent conductors of heat, and consequently can handle higher brake loads and temperatures. Semi-met
pads are commonly used on front-wheel drive disc brake applications and other applications that require increased brake
performance. Asbestos or NAO pads should never be substituted for an application that requires semi-mets.
The names for the components in a drum brake that support the linings. In self-energizing drum brakes, the forward shoe is
often referred to as the primary shoe, while the rear shoe is called the secondary shoe.
An unvented type of rotor with no cooling fins between its faces.
STAR WHEEL ADJUSTER
A threaded adjuster mechanism in a drum brake that moves the brake shoes further apart so the linings will be closer to the
See Traction Control
An enhancement of an existing ABS system that prevents wheel spin while accelerating on wet or slick surfaces. It uses the
same wheel speed sensors to monitor wheel speed during acceleration, but requires some additional control solenoids and a
pump to apply braking pressure to control wheel spin. The traction control system brakes the drive wheel that's starting to spin
to shift torque to the opposite drive wheel that still has traction. Most traction control systems only operate at speeds up to
about 30 mph.
Additional control strategies that some traction control systems use to limit wheel spin include reducing the throttle opening,
upshifting the transmission, retarding spark timing and deactivating fuel injectors.
A numeric code that's associated with a specific type of problem or fault that may be present in an ABS system. In ABS
systems with self-diagnostic capability (not all have this feature), the 0ABS control module runs a self-check when the vehicle
is started and driven to make sure all the ABS circuits are functioning properly. It also monitors system operation while the
vehicle is being driven. If it sees a problem (no speed sensor signal when there should be one, loss of voltage in a circuit, etc.),
it triggers a trouble code and is logged in the control module's memory. At the same time, the ABS warning light is illuminated
to alert the driver of a potential problem. The ABS system is also deactivated. The trouble code (there can be more than one)
can then be retrieved by a technician using either the flash code technique or a scan tool to diagnose and repair the problem.
A procedure for bleeding the brakes that uses a special vacuum tool to pull fluid through the lines and out the bleeder screws.
It works much the same as power bleeding and does not require special adapters for the master cylinder or holding the
metering valve open.
The vacuum diaphragm unit in a power brake system that multiplies the force exerted on the brake pedal to the master
cylinder. Vacuum is supplied by a line to the engine intake manifold.
A rotor with cooling fins between its faces. The cooling fins allow faster heat dissipation for improved braking ability and longer
The component in a drum brake that forces the shoes out against the drums. Hydraulic pressure pushes the pistons outward,
which push the shoes out to apply the brakes. Leaks are a common problem, and can be repaired by either honing and
rebuilding the wheel cylinder or replacing it.
WHEEL SPEED SENSOR
A sensor that monitors wheel speed. Some ABS systems have one speed sensor foe each wheel. Others use a common
sensor mounted in the transmission or differential to read the average speed of both rear wheels. The sensor pickup consists
of a magnet surrounded by a coil of wire. The signal is generated by a notched ring that rotates under the sensor. As the teeth
pass through the sensor's magnetic field, they generate an alternating current voltage signal in the pickup windings. The
frequency of the signal is proportional to the speed of the wheel(s).