Like fingernails scraping across a blackboard, disc brake squeal is enough to make anybody's hair stand on end. For some neurological reason that is not fully understood, human beings react negatively to high-pitched squeals like crying babies, sirens and screeching breaks. So if your brakes are squealing, you want the noise to go away.
Brake squealing is produced by high-frequency vibration in the brakes. With disc brakes, vibrations can occur between the pads and rotors; the pads and calipers; the calipers and mounts; and/or within the rotors themselves. With drum brakes, the vibrations can originate between the shoes and the backing plates, and/or within the drums.
The noise is not dangerous as long as there is no metal-to-metal contact, the brakes are working properly and there is adequate lining thickness. But, it sure can be annoying. So, to get rid of it, you first have to figure out what is causing the brake noise.
DISC BRAKE SQUEAL
Complaints about brake squeal became a problem when front-wheel drive and semi-metallic brakes arrived on the scene in the 1980s. Semi-metallic pads are harder than their asbestos counterparts, and thus are more apt to chatter and squeal if there are any irregularities or roughness on the rotor surface, or if you notice looseness between the pads and calipers.
Some types of caliper designs are more apt to be noisy than others. The pads in these calipers may not be held as tightly and/or the caliper itself may move around a lot when the brakes are applied. And, as we said earlier, the greater the play in the system, the greater the tendency to make noise. That's why some new car dealers try to dismiss the problem by telling their customers some noise is "normal", leaving the customer no alternative but to live with the problem or to get it fixed by somebody else.
Trying to fix a squeal problem the wrong way can often make the problem worse. If somebody does a quick brake job and replaces the brake pads but doe snot resurface the rotors, the result can be an even louder squeal. The same can happen if the rotors are resurfaced incorrectly, too quickly or with dull tools. Excessive rotor runout can also cause problems.
DRUM BRAKE NOISE
One of the leading causes of brake squeal in drum brakes is poor contact between the shoes and drum. Heel and toe contact between the shoe and drum is often the culprit, and the cure is to either replace the shoes with new ones or to resurface the drum slightly to increase its inside diameter. New shoes are ground with a slight eccentric to compensate for drum wear.
This moves the point of contact away from the ends of the shoes toward the middle. In the old days, mechanics used to arc shoes to match their shape to the drum. But, with the concerns about asbestos, shoe grinding is pretty much a thing of the past (although some say it will make a comeback as more and more new cars switch to non-asbestos linings on their drum brakes).
INSPECTING THE BRAKES
Open the hood, and check the brake fluid level and its appearance. A low level may indicate a leak or worn linings. Discoloration indicates moisture contamination and the need for a fluid change. An electronic tester or chemical test strips can be used to check the level of moisture contamination in the fluid.
Apply the brakes, and start the engine. Does the pedal drop slightly? It should because it indicates a good vacuum booster. No boost may indicate a leaky booster diaphragm or vacuum connection. How does the brake pedal feel? Is it firm? A soft or mushy-feeling pedal usually indicates air in the lines or leaks. A pedal that slowly sinks is a classic symptom of a worn master cylinder. Is the amount of pedal travel normal? A low pedal may indicate worn linings, the need for adjustment, defective/frozen drum brake adjusters or a low fluid level. Do the brake lights come on when you step on the pedal? No lights may indicate a defective or misadjusted brake light pedal switch or burned out bulbs in the tail lights.
On ABS-equipped vehicles, turn the ignition on to verify that the ABS warning light circuit works. The ABS light should come on for a few seconds, then go out if everything is fine. No light? Then you have found a bulb that needs replacing or a wiring problem. If the light comes on and remains on (does not go out), then further diagnosis will be required to find out what's wrong with the ABS system. On some ABS systems, faults may have occurred that may not be serious enough to cause a continuous ABS warning light. These may be stored in the ABS module memory as "non-latching" or "soft" fault codes. Don't ignore ABS codes because they may be a clue that more serious problems will be forthcoming.
Apply the parking brake. Does the pedal or handle work smoothly? Is it adjusted properly? Does the brake light come on? No brake warning light may indicate a bad bulb or defective or misadjusted parking brake switch. Does the parking brake hold the vehicle? Put the transmission into gear with the parking brake applied. If it fails to hold the vehicle, it needs adjusting. Now release the parking brake. Failure to release fully means the linkage, cables or locking mechanisms need attention.
Take a short test drive. DO NOT drive the vehicle if the brakes have failed, if there is insufficient pedal travel or firmness to stop the vehicle safely, or there is a serious fluid leak. While driving, apply the brakes several times to check for noise, pull to either side or grabbing. Also check for drag when the brakes are released. Note pedal feel, especially any pulsation that would indicate warped rotors. If possible, do a panic stop to check for ABS operation.
Remove a front wheel and measure the thickness of the brake pads. If worn down to minimum specifications or if wear indicators are making contact with the rotor, new linings are needed. If the pads are still above specs, they should probably be replaced anyway if they are near the end of their service life or if they are noisy. Also, note the condition of the rotors. Deep scratches or grooves indicate a need for resurfacing. Measure runout and parallelism, too. If out of specs, resurfacing or replacement is needed. Are there discolored spots, heat cracks or warpage? These symptoms may also indicate a need for rotor resurfacing or replacement.
Note the condition of the calipers and caliper mounts. Also note whether or not the pads are worn evenly. Uneven pad wear can be caused by corrosion on the caliper mounting guides or keyway.
Pull a drum, and inspect the drum surface, brake shoes, hardware and wheel cylinder. If the shoe linings are at or below minimum specifications, new shoes are needed. If the linings are still above minimum specs but are getting thin, new shoes are recommended to extend the life of the brakes.