BASIC BRAKE SYSTEM TROUBLESHOOTING
There comes a time in the life of every vehicle when the brakes need attention. Brakes are a wear item so eventually the
linings wear out. But sometimes other things go wrong with the brakes, which means you have to troubleshoot the system to
figure out what needs fixing. So let's proceed to some common and some not-so-common problems you're apt to encounter.
BRAKE WARNING LIGHT
If the light is on all the time, it could mean you simply forgot to release the parking (emergency) brake. The brake warning light
remains on when the brake is set as a reminder. But if the light stays on when you release the parking brake, you've got a
In some cases, the brake warning light may remain on if the switch on the parking brake pedal or lever is misadjusted or
sticking. Nothing is wrong with the brakes and a simple adjustment is all that's needed to solve the problem.
A light that remains on, or comes on when you apply the brakes, though, usually means low brake fluid and/or a loss of
hydraulic pressure in one side of the brake system. In either case, the fluid level in the master cylinder should be checked.
Adding brake fluid to the master cylinder reservoir may temporarily solve your problem. But if there's a leak in the system, the
new fluid will soon be lost and the warning light will come back on.
Brake fluid leaks are serious because they may cause the brakes to fail! So don't drive your vehicle until the problem has been
identified and fixed. Leaks can occur in brake hoses, brake lines, disc brake calipers, drum brake wheel cylinders or the master
cylinder itself. Wet spots at hose or line connections would indicate fluid leakage and a need for repairs.
Leaking brake fluid can contaminate the brake linings, causing them to slip or grab. The uneven braking action that results may
cause the vehicle to veer to one side when the brakes are applied. Brake shoes or pads that have been contaminated with
brake fluid cannot be dried out and must be replaced.
ABS WARNING LIGHT
If your car or truck is equipped with antilock brakes (ABS), a second warning light is usually provided to warn you if a problem
occurs in your ABS system. The ABS lamp comes on when the ignition is turned on for a bulb check, and should go out after
the engine starts. If the ABS warning light remains on or comes on while driving, it indicates a fault has occurred in the ABS
What happens next depends on the nature of the fault. On most applications, the ABS system disables itself if the ABS
warning light comes on and remains on. This should have no effect on normal braking. But it also means your ABS will NOT be
available in an emergency situation or when braking on a wet or slick surface.
CAUTION: If the brake warning light also comes on and remains on while the ABS warning light it on, it signals a serious
problem. Your vehicle may not be safe to drive. The brakes and ABS system should be inspected immediately to determine
the nature of the problem!
If the ABS light comes on momentarily then goes out, the nature of the problem is probably minor and the ABS system will
usually remain fully operational. Some vehicle manufacturers call this kind of fault a "nonlatching" fault (meaning it isn't serious
enough to disable the ABS system).
Regardless of the type of fault that occurred to trigger the ABS warning lamp, a special "code" is recorded in the ABS module's
memory to aid in diagnosing the problem. On some vehicles this code can be retrieved by putting the ABS system into a
special diagnostic mode. The code is then flashed out through the ABS warning lamp. The code number refers to a diagnostic
chart in a service manual that must be followed to pinpoint the faulty component. On other applications, a scan tool must be
plugged into the vehicle's diagnostic connector to read out fault codes.
Diagnosing ABS problems requires a fair amount of knowledge and expertise (as well as special equipment in many
applications), so this job may be better left to a competent professional.
DISC BRAKE SQUEAL
High pitched squeals can be very irritating but do not usually mean anything is wrong with your brakes. Even so, you should
always investigate a noise problem to determine what's causing it.
Squeals are usually caused by vibrations between disc brake pads, caliper and rotor. The intensity of the squeal depends on
the hardness of the brake linings (semi-metallic pads in front-wheel drive cars are the worst offenders), the rigidity of the
caliper mounting, rotor finish and vehicle speed.
Possible causes include:
* Glazed brake pads. When pads become glazed (very smooth and glassy), they tend to squeal. Replacing the pads may
temporarily eliminate the noise, but unless the underlying problem that caused the pads to glaze in the first place is diagnosed
and corrected, the noise will likely return.
The most common cause of pad glazing is overheating, which may be the result of brake drag caused by a sticky caliper or a
problem in the hydraulic system (such as a restriction in a brake line that prevents the brakes from releasing fully, or a
misadjusted stop light switch that is creating residual pressure in the master cylinder). A sticky or frozen caliper will often cause
the pads to wear unevenly depending on the design of the brake system. If the inside pad is worn more than the outside pad
on a single piston floating caliper, for example, it would tell you the caliper is hanging up and needs attention. To prevent this
kind of problem, make sure the caliper slides and bushings are clean, rust-free and properly lubricated with high temperature
* Missing insulator shims. Many disc brakes have shims on the backs of the pads to dampen vibrations. If someone else
relined your brakes and forgot to install the required shims, the pads may be noisy. You can buy replacement shims if the
shims are missing, or apply a noise control compound or brake grease to the backs of the pads (never the front!) to help
cushion the pads and dampen vibrations between the pads and calipers.
* Worn disc brake hardware. The mounting hardware (springs, shims, keyways, clips, pins, bushings, etc.) that align the caliper
and pads with the rotor may become worn and loose with age. This can lead to improper alignment and noise. Misalignment
can also cause hot spots and glazing. The cure here is to replace the worn hardware.
* Outboard pad looseness. Many outboard disc brake pads are held in place by retaining ears built into the steel backing plate.
These ears must be tight and secure to prevent the pad from moving and becoming improperly aligned. If the pad is loose and
making noise, bend the ears so the pad will be held securely.
* Wear indicators touching the rotor surface. Many disc brake pads have small wear indicator tabs. The tabs produce a loud
squealing noise when the pads wear down and the tab starts to rub against the rotor. This is your clue that it's time to replace
the pads. Don't ignore the warning because if you wait too long, you may chew up your rotors.
* Rough rotor finish. If the rotors are too rough, the pads will chatter and squeal. Resurfacing the rotors to restore a like-new
finish may be all that's needed to eliminate the noise. But if your vehicle has semi-metallic disc brake pads, sanding the rotors
with #120 or #150 grit sand paper after they've been turned to produce a smooth nondirectional swirl finish is a good idea to
assure quiet braking.
SCRAPING NOISES FROM THE BRAKES
Metallic scraping noises usually mean repairs are needed. Possible causes of this kind of noise include:
* Worn pads or shoes. If riveted pads or shoes are worn down to the rivet heads, or if bonded pads or shoes are worn down to
the backing plates, the metal-to-metal contact that results with the rotor or drum will produce a metallic scraping or grinding
noise when the brakes are applied. Not only does this create an unsafe braking condition, but it can be very damaging to rotors
and drums. In addition to replacing the worn linings, it will probably be necessary to resurface or replace the rotors and/or
* Improper caliper alignment. If the caliper is misaligned with respect to the rotor and spindle, you may hear metallic scraping
sounds. The cure is to realign the caliper by installing new disc brake hardware. Anything that is damaged or worn should be
replaced. If the caliper itself has broken mounting ears or badly corroded or worn guides, then it will have to be replaced.
* Rust or mud on caliper housing or rotor edge. Inspect the brake components and remove any debris or buildup of rust that's
found. The caliper and/or rotor may have to be replaced if rusted to the point where it interferes with proper operation.
* Loose or broken drum hardware, or debris inside the drum. Loose "junk" inside the drum will rattle and scrape as the drum
turns, and may even wedge itself between the shoes and drum causing the brake to bind. If your hear noise from the vicinity of
the drum, therefore, you'll have to pull the drum to investigate.
* Loose or worn wheel bearings. This may also be accompanied by steering looseness or wander.
WARNING: Do not ignore noisy wheel bearings. Noise indicates roughness or damage that may cause sudden failure of the
bearings and loss of a wheel!
Raise the wheel off the ground and feel for roughness as you turn it by hand. Also, try to wobble the wheel back and forth to
feel for looseness. The only way to be absolutely sure, though, is to remove and inspect the wheel bearings, then replace or
clean and repack them as needed. You can't do this with sealed wheel bearings, so if there's any play or roughness, the
bearing assembly or hub needs to be replaced.
DISC BRAKE RATTLE
A rattle that's heard at low to moderate speed, usually when encountering bumps, may be caused by the following:
* Anti-rattle springs or clips missing or installed improperly. replace the anti-rattle springs or hardware and make sure the pads
are positioned properly.
* Clearance problem between pads and rotor. Pads that are not the proper thickness for the application may cause a noise
problem. The cure is to replace them with ones that are right for the application.
* Frozen caliper. If a floating caliper is hung up and cannot center itself over the rotor, it may produce a rattling noise when the
brakes are applied. Remove the caliper, clean and lubricate all moving surfaces, and replace the slides or bushings.
* Improper rotor finish. Disc brake chatter can be caused by rotors that have been improperly finished. A brake lathe cuts
grooves in a rotor that resemble the grooves in a phonograph record. If the ridges are too pronounced, they'll grab the pads
and make them chatter up and down whenever the brakes are applied. The cure here is to have the rotors refinished.
A pulsating brake pedal, which may be accompanied by a shuddering or jerky stop usually means you have warped rotors or
out-of-round drums. But the same symptom can sometimes be caused by loose wheel bearings, a bent axle shaft or loose
The faces of a rotor must be parallel (within .0005 inch on most vehicles) and flat (no more than about .002 to .005 inches of
runout) otherwise you may feel a pulsation in the brake pedal when you step on the brakes.
You can usually spot a warped rotor by the telltale glazed or discolored patches on its faces. Resurfacing the rotor to restore
the faces will usually eliminate the pulsation, but hard spots often return so you may have to replace the rotors to get rid of the
problem permanently. If a rotor is bent, replacement is your only option.
Warped rotors can be caused by dragging brakes, overheating and/or unevenly tightening the lug nuts -- which is why you
should always use a torque wrench, not a four-way lug or an impact wrench to final tighten the lug nuts.
A drum can sometimes be warped out-of-round by applying the parking brake when the brakes are hot. As the drum cools, the
force of the shoes causes it to distort out of shape.
A catchall phrase that covers a lot of territory, erratic (not "erotic") braking generally means uneven brake action. It can also
encompass grabbing, pulsating and pulling. Contaminated brake linings (brake fluid or grease), misadjusted drum brakes,
loose or binding calipers, a faulty metering or proportioning valve, mismatched brake linings (a higher friction lining or different
material on one wheel or one side), restricted brake lines, loose front end parts or wheel bearings, or even mismatched tire
inflation pressures may make your brakes behave strangely.
Pulling to one side when the brakes are applied signals uneven braking action. This happens when the brakes on one side are
not working as well as those on the opposite side. The vehicle pulls towards the side that creates the most friction. Oil or
grease contaminated linings on one side, misadjusted brakes, a bad wheel cylinder or caliper, a dragging brake, even loose
wheel bearings, front end parts or underinflated tires can cause a brake pull.
Oil, grease or brake fluid contaminated linings are the most frequent cause of brake grab followed by scored drums or rotors. A
loose caliper can also interfere with smooth braking.
Oil can come from a leaky rear axle seal and grease from a leaky wheel bearing seal. Never reuse an old seal when repacking
or replacing wheel bearings.
A hot wheel, a sudden appetite for brake linings and/or a drop in fuel economy are all symptoms of brake dragging. Age and
corrosion are two factors that can bring about dragging so check for things like weak or broken retracting springs on drum
brakes, a frozen or corroded caliper piston, a floating caliper that has ceased to float because the mounting pins or bushings
are corroded tight (uneven wear between the inner and outer pads is a dead giveaway to this problem), overextended drum
brake self-adjusters, an emergency brake cable that fails to fully release, or a defective quick takeup valve in the master
cylinder on a late model GM product.
An overadjusted stop light switch can sometimes prevent the brake pedal from coming all the way out which leaves a residual
pressure in the system and causes the brakes to drag. On a diagonally split hydraulic system, the right front caliper is
frequently the one that drags. The left rear drum brake has retracting springs to resist the residual pressure but there's nothing
to prevent the caliper piston from pushing out.
A low brake pedal that has to be pumped repeatedly to bring a vehicle to a stop may be due to a low fluid level, drum brakes
that need adjustment or air in the lines.
The first thing you should check is the fluid level in the master cylinder reservoir. If the level is low, there's a leak somewhere in
the brake hydraulics that must be found and repaired. Adding fluid will only cure the symptom, not the cause, and sooner or
later the level will be low again creating a dangerous situation. So check for leaks around the master cylinder, wheel cylinders,
brake calipers, rubber brake hoses and steel brake lines.
If the fluid level is okay, check the adjustment of the rear brakes (assuming the car is a late model with discs up front and
drums in the rear -- if it has drums all the way around, check the front ones first). The shoes should be close enough to the
drums to produce just a hint of drag when the wheels are rotated by hand. An excess of slack probably means the
self-adjusters are either frozen or fully extended. Refer to a shop manual for the brake adjustment procedure on your vehicle. If
adjusting the drum brakes fails to eliminate the low pedal, you'll have to pull the wheel and drum to free-up or replace the
adjusters and/or replace the worn brake shoes.
If the fluid reservoir is full and the brakes are properly adjusted but the pedal is low (or feels spongy), there is probably air in
the brake lines. Air is compressible, so every time you step on the pedal, the bubbles collapse instead of transferring pressure
to the brakes. The cure here is to "bleed" the brake lines.
Bleeding the brakes involves flushing the lines with fresh hydraulic fluid. The basic procedure for bleeding brakes is to start at
the wheel furthest from the master cylinder. You'll need a piece of clear plastic tubing about 18 inches long. One end of the
tubing should fit tightly over the small bleeder screw on the back of the wheel cylinder or brake caliper. The other end should
be inserted into a clear glass or plastic container. Loosen the bleeder valve on the wheel to be bled, then slowly push the brake
pedal all the way to the floor to force fluid through the line. Release the pedal slowly, wait a few seconds, then repeat until
clean, clear fluid and no bubbles are visible in the clear plastic tubing. Retighten the bleeder screw, remove the plastic tubing
and proceed to the next wheel. Make sure you add fresh fluid to the master cylinder while bleeding the brakes so the fluid
reservoir doesn't get too low and allow air to be pumped into the lines.
Normally you bleed the rear brakes first and then the front -- except on front-wheel drive cars with diagonally-split hydraulic
systems. On these vehicles, you bleed opposing wheel pairs doing the right rear and left front pair first followed by the left rear
and right front brakes. On late model GM cars, the quick takeup valve in the master cylinder takes about 15 seconds to reseat
between pumps. Pump it too quickly while bleeding the system and you won't get any pedal. If pressure bleeding, remember to
open the metering valve otherwise you won't get all the air out of the front brake lines. Refer to a shop manual for the specific
brake bleeding procedure for your vehicle.
A brake pedal should be firm with a predictable amount of resistance. But when it feels squishy you can bet there's probably air
in the lines and a leak somewhere. Fixing the leaks and bleeding the system should eliminate the soft pedal. Sometimes,
however, the cause is something like a ballooning brake hose, a worn out drum, warped brake shoes or new shoes that have
not yet seated.
PEDAL SINKS TO FLOOR
There's nothing more assuring than a nice firm brake pedal -- and nothing more unnerving than a pedal that slowly sinks to the
floor while you're pressing on it. This condition is most noticeable when sitting at a stop light. By the time the light turns green,
the pedal may have dropped several inches or may require pumping to keep your vehicle from creeping ahead. This is usually
caused by one of two things: a worn master cylinder or a leak in the hydraulic system. If a leak's responsible for the fading
pedal, the system will soon lose all its fluid and the brakes will cease to work at all.
The first thing to check is the fluid level. If low look for a leaky brake hose, brake line, wheel cylinder or caliper. A full fluid
reservoir and no visible leaks means the master cylinder is worn out and needs to be rebuilt or replaced.
Excessive pedal effort may be due to a faulty power booster if you have power brakes. The booster may be defective or the
loss of assist may be because of insufficient vacuum due to a leaky hose, vacuum reservoir or a vacuum leak elsewhere.
To check the vacuum booster, pump the brake pedal several times with the engine off to bleed off any vacuum that may still be
in the unit. Then hold your foot on the pedal and start the engine. If the booster is working, you should feel the amount of effort
required to hold the pedal drop and the pedal itself may depress slightly. If nothing happens, check the vacuum connections
with the booster unit and run a vacuum check on the engine (a minimum of 17 inches is needed). No vacuum leaks but still no
power assist means you need a new booster.
Sometimes a faulty check valve will allow vacuum to bleed out of the booster causing a hard pedal when the brakes are
applied. This can be checked by starting the engine (to build vacuum) then shutting it off and waiting four or five minutes. Then
try the brake pedal to see if there is any power assist. You should get at least a couple of assists if the check valve is holding.
If not, replace the check valve.
If your vehicle has Hydro-Boost, a hard pedal can be caused by a loose power steering pump belt, a low fluid level, leaks in the
power hoses, or leaks or faulty valves in the Hydro-Boost unit itself -- which require replacing the booster.
A hard pedal on a vehicle without power brakes can be caused by glazed or worn linings, grease or oil on the brake linings, wet
brakes, seized or frozen wheel cylinders and/or brake caliper pistons, a defective master cylinder or, on rear-wheel drive
applications, the use of semi-metallic linings which typically require more pedal effort than asbestos or NAO linings (especially
NO PARKING BRAKE
If you pull the parking brake lever or step on the pedal and the brake fails to hold your vehicle on a hill, maybe all you need is a
simple adjustment. As a rule, most hand levers and pedals should travel only about 4 or 5 "clicks" when properly adjusted. If
the handle or pedal travels too far, it may not pull the linkage and cables tight enough to set the brake.
If you feel little or no resistance when applying the parking brake, the equalizer yoke or cables may be broken.
If the parking brake handle or pedal is frozen, rusted cables are the most likely cause. If you can't free them up with
penetrating oil, you'll have to replace them.