Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.

Some brands of semi-metallic pads are in herently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.

Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or antirattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.

Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.

If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).

Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.

If the brake warning light is on, you most likely have a fluid leak. Your vehicle may not be safe to drive in this condition! You should have the brakes inspected as soon as possible to determine where the fluid is leaking (usually a hose, brake line, brake caliper or wheel cylinder) so the necessary repairs can be made.

If the brake warning light is not on, it does not necessarily mean you do not have a leak. The warning light only comes on when there's been enough fluid loss to create a pressure differential between the two sides of the hydraulic system that actually apply the brakes.

The brake system is divided into two hydraulic circuits. On most rear-wheel drive vehicles, it is divided so one circuit applies the front brakes and the other applies the rear brakes. On front-wheel drive cars and minivans, the system is usually split diagonally. One circuit works the right front and left rear brake, and the other works the left front and right rear brake. This is done for safety purposes so if one circuit loses all its brake fluid and fails, the vehicle will still have one remaining circuit to apply two wheel brakes.

A quick way to check for leaks in either circuit is to simply check the fluid level in the master cylinder reservoir. The reservoir is divided into two chambers (one for each brake circuit). If one chamber is unusually low or empty, there's a leak somewhere in that circuit. The brakes should then be inspected to check for fluid leaks. Wet spots around hose or line connections, or fluid leaking from a disc brake caliper or drum wheel cylinder would indicate a serious problem that needs immediate attention.

If the brake warning light is not on and there are no apparent leaks, then the master cylinder may be worn or leaking internally allowing the pedal to slowly sink when pressure is applied to it. This type of condition will be most noticeable when holding constant pressure against the brake pedal at a stop light. If the pedal sinks or requires pumping to keep the car from creeping ahead, the master cylinder needs to be replaced.

On some vehicles with rear-wheel antilock brake systems (ABS), it's also possible that a leak in the ABS unit may cause a similar sinking pedal condition.

The faces of a disc brake rotor must be parallel (within .0005 inch on most cars) and flat (no more than about .002 to .005 inches of runout) otherwise it will kick the brake pads in and out when the brakes are applied, producing a pulsation or vibration that can be felt in the brake pedal as the rotor alternately grabs and slips.

You can often see warpage in a brake rotor by simply looking at it. If the rotor has telltale glazed or discolored patches on its face, chances are it is warped. Measuring it with a dial indicator and checking it for flatness with a straight edge will confirm the diagnosis.

Resurfacing the rotor to restore the faces will usually eliminate the pulsation (unless the rotor is bent or is badly worn and has started to collapse in which case the rotor must be replaced). But it may only do so temporarily because of metallurgical changes that take place in the rotor. Hard spots often extend below the surface of the rotor. Resurfacing will restore the surface, but the hard spot may reappear again in a few thousand miles as the rotor wears. For this reason, replacing warped rotors rather than resurfacing them is recommended.

Pedal pulsation caused by drum warpage isn't as common, but it can happen. 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 the drum to distort.

What causes a rotor to warp? Overtorquing or unevenly torquing the lug nuts with an impact wrench is a common cause. For this reason, most experts recommend using a torque wrench to tighten lug nuts when changing a wheel. There are also special torque-limiting extension sockets called "Torque Sticks" that can be safely used with an impact wrench to accurately tighten lug nuts. But a plain impact wrench should never be used for the final tightening of the lug nuts because most provide no control whatsoever over the amount of torque applied to the nuts.

Overheating can also cause rotors to warp. Overheating may be the result of severe abuse or dragging brakes. Defects in the rotor casting, such as thick and thin areas can also cause uneven cooling that leads to warpage. Hard spots in the metal due to casting impurities can be yet another cause.

If the pedal feels "soft" or "spongy" instead of firm, there's probably air in the system. This will require "bleeding the brakes" to remove air from the lines, calipers and wheel cylinders.

The first thing that should be checked is the fluid level in the master cylinder reservoir. If the level is low, there's a leak somewhere in the hydraulic system 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, the adjustment of the rear brakes should be checked next (assuming the vehicle has drum brakes in the rear -- if it has drums all the way around, check the front drums first, then the rear). 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.

If adjusting the drum brakes fails to eliminate the low pedal, the wheel and drum will have to be removed so the adjusters can be freed up or replaced, and/or so the worn brake shoes can be replaced.

If the vehicle has rear disc brakes, the adjusting mechanism in the rear caliper pistons that maintain the correct pad-to-rotor clearance may be corroded, frozen or worn out. In most cases, the piston assemblies cannot be rebuilt and must be replaced.

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 following the factory recommended sequence.

Brakes are usually bled in a specified sequence (always refer to a shop manual for the exact procedure for your vehicle). Usually the rear brakes are bled first, then the ones up front on most rear-wheel drive cars and trucks. But on front-wheel drive cars and minivans, the hydraulic system is split diagonally so the brakes are bled in opposite pairs (right rear and left front, then left rear and right front). Following the proper sequence is important so air doesn't remain trapped in the lines.

On some late model cars with quick take-up master cylinders, the quick take-up valve takes about 15 seconds to reseat after the brake pedal has been depressed. If the pedal is pumped too quickly while manually bleeding the system, you may never get the pedal to firm up. Most professionals use pressure bleeding equipment to bleed the brakes because it is faster and easier.

Answer: You need a "brake job" when your brake linings are worn down to the minimum acceptable thickness specified by the vehicle manufacturer or the applicable state agency in areas that set their own requirements. The only way to determine if new linings are required, therefore, is to inspect the brakes.

You may also need a brake job if you're having brake problems such as grabbing, pulling, low or soft pedal, pedal vibration, noise, etc., or if some component in your brake system has failed. But if the problem is isolated to only one component, there's no need to replace other parts that are still in perfectly good working order.

There is no specific mileage interval at which the brakes need to be relined because brake wear varies depending on how the vehicle is driven, the braking habits of the driver, the weight of the vehicle, the design of the brake system and a dozen other variables. A set of brake linings that last 70,000 miles or more on a car driven mostly on the highway may last only 30,000 or 40,000 miles on the same vehicle that is driven mostly in stop-and-go city traffic.

As a rule, the front brakes wear out before the ones on the rear because the front brakes handle a higher percentage of the braking load -- especially in front-wheel drive cars and minivans. So many service facilities advertise brake job "specials" that replace the linings on the front brakes only. Doing the front brakes only is okay and can save you money as long as the rear brakes are in good condition. But if the rear brakes need attention, they should be relined too.

A person typically goes in expecting to spend a nominal amount for a brake job, but usually ends up spending considerably more because the brakes need more than the minimum amount of work to restore them to like-new condition. The price of a brake job depends entirely on the work that needs to be performed. So any advertised special is not a firm price, but only an estimate of the least amount of money it might cost you to get your brakes fixed. A price should not be quoted until after the brakes have been inspected. Then and only then can an accurate determination be made of the parts that actually need to be replaced.

Some brake jobs may also include new hardware for the drums (recommended), and rebuilding or replacing the wheel cylinders and calipers (also recommended). But because of the added expense, these items may not be included in the package price or may only be done if the brake system really needs them (as opposed to doing them for preventative maintenance).

Hardware includes things like return springs, holddown springs and other clips and retainers found in drum brakes. It may also include bushings, pins and clips on disc brake calipers. Springs lose tension with age and exposure to heat. Most experts recommend replacing the hardware when relining drum brakes to restore proper brake action. If weak springs are reused, the shoes may drag against the drums causing accelerated shoe wear, a pull to one side, brake overheating and possible drum warpage. Other hardware that is badly corroded or faulty (such as the self-adjusters) may prevent the shoes from maintaining the correct drum clearance (which increases the distance the brake pedal must travel as the shoes wear), or the parking brake from functioning properly.

It's important to note that not all replacement linings are the same. There are usually several grades of quality in pads and shoes (good, better and best). The difference is in the ingredients that are used to manufacture the pads and shoes. The less expensive ones may cost less initially and save you a few dollars on your total bill, but you may not be happy with the way they wear and perform. All brake linings must meet minimum government safety standards. Even so, the cheaper grade of pads and shoes do not last as many miles as the premium grade of replacement linings, nor do they brake as effectively. They usually have a greater tendency to fade at high temperature and may increase the vehicle's stopping distance somewhat. Noise may also be a problem with cheap linings. The best performance and value for your money, therefore, is with the best or premium grade. Choose these when the brakes are relined.

Surface condition. The most important criteria. Scoring, pitting or other minor surface imperfections should be cleaned up by resurfacing. Resurfacing is also recommended if the brakes have been noisy. If a rotor or drum is cracked, however, it must be replaced.

Lateral runout on rotors. This must be measured with a dial indicator against the face of the rotor. If rotor wobble exceeds OEM specs, reindexing it on the hub (if possible) may help reduce runout. But a better solution here is to resurface the rotor on the vehicle using a special "on-car" lathe. This type of equipment cuts the rotor true to its axis of rotation.

Rotor warpage. Variation in the thickness of the rotor or uneven spots on either rotor face will cause the brake pedal to pulsate or shudder when the brakes are applied. Flatness can be checked by placing a straight edge against both faces of the rotor. Thickness must be checked with a micrometer at six or more points around the rotor. If parallelism between rotor faces exceeds OEM specs (generally about .0005 in.), or if the rotor is warped or has hard spots (which are often discolored blue or black), the rotor should be resurfaced or replaced. Hard spots that develop from overheating or uneven tightening of lug nuts can create raised areas on the surface that often extend below the surface. The metallurgical changes in the rotor often cause the hard spots to return after a few thousand miles so replacing the rotor may be the best long-term fix.

Rotor thickness and drum diameter -- If a rotor is close to or at the minimum thickness specification (which is stamped on the rotor or may be found in a brake service reference book), it is too thin to be resurfaced and must be replaced. The same goes for drums, except the critical dimension here is the drum's inside diameter (the maximum or discard diameter spec is stamped on the drum or listed in a reference book).

When a rotor or drum is resurfaced, the least amount of metal possible should be removed to maximize the remaining service life of the part. It's also a good idea to sand rotors after they've been turned with sandpaper to create a smooth, nondirectional or crosshatch finish. Sanding helps "plateau" the surface by knocking off the sharp peaks left by a lathe. Some brake lathe manufacturers say this final step isn't necessary because their equipment leaves such a smooth finish. Maybe so. But sanding is still a good way to ensure proper pad seating and minimize the chance of noise with semi-metallic pads.

Finally, rotors and drums should always be cleaned with alcohol or brake cleaner before they are reinstalled. Resurfacing leaves a lot of metallic debris on the surface which can embed itself in the new brake linings and cause noise problems. Even if the rotors or drums have not been resurfaced, cleaning is recommended to remove dirt and grease (which can contaminate new linings and cause uneven braking or grabbing).

As for replacing or rebuilding the disc brake calipers when the brakes are relined, this too isn't absolutely necessary -- but not doing so may be asking for trouble later. A caliper that's leaking brake fluid, is damaged, has a frozen piston, or is causing uneven pad wear obviously needs to be rebuilt or replaced. But what about a caliper that isn't leaking and seems to be working fine? Those who subscribe to the "don't fix it unless it's broken" philosophy of brake repair think they're saving time and money by leaving the calipers alone. Maybe so. But most brake experts say it makes sense to go ahead and do the calipers when the brakes are relined. Here's why:

Brake calipers, like any other mechanical component, wear and corrode with age. Every time the brakes are applied, the back-and-forth motion of the caliper pistons produces a slight amount of wear. At the same time, moisture is building up in the fluid. After several years of service, the fluid may contain as much as 3% water -- which is enough to produce visible corrosion in the caliper bores and on steel pistons. As the surface of the pistons become rough, they scour the seals with every application of the brakes. Eventually this will lead to fluid leaks and pad contamination.

So even if a caliper isn't leaking, it's still aging inside. Rubber piston seals and dust boots harden and become brittle over time. One of the jobs the piston seals do besides keep the fluid where it belongs is to help retract the pistons when the brakes are released. When a piston moves out, it twists the square cut seal slightly. This helps pull the piston back when the brakes are released to keep the pads from dragging against the rotors. But as a seal ages and loses elasticity, it doesn't do as good a job of pulling back the piston. Consequently, the pads start to drag resulting in reduced fuel economy, a possible brake pull and/or accelerated or uneven pad wear. When boots get old, they often crack or split, allowing dirt and water to enter the piston bore area. The result can be accelerated seal wear, piston corrosion and sticking. What's more, if the pistons are pushed back in to accept new pads any dirt that's found its way behind the boots will be shoved back into the caliper bores. This too, can contribute to sticking, binding and wear. So there are valid reasons for rebuilding or replacing the calipers when the brakes are relined.

The same arguments that hold true for disc brake calipers also apply to the wheel cylinders in drum brakes. As long as they're not leaking, sticking or damaged, you can leave them alone and take your chances. Or, you can do the wise thing and rebuild or replace them when you reline the brakes for preventative maintenance.

To remove air bubbles that may have entered the system while repairs were being made, because of a leak or because the fluid level got too low. The air must be removed because it is compressible and can prevent a full, firm pedal. The individual brake lines must be bled in a specified sequence (which varies from vehicle to vehicle depending on the design of the brake system) to remove all the air from the lines. On some ABS-equipped vehicles, special bleeding procedures may be required (which also requires special equipment in some cases such as a scan tool to cycle the ABS solenoids). The brakes can be bled manually by attaching a piece of clear tubing to the bleeder screw on each caliper and wheel cylinder, opening the screw and manually stroking the brake pedal to force fluid through the lines, or with power bleeding equipment. Most professionals use power bleeding equipment because it's faster and easier.

To remove moisture contamination. Brake fluid needs to be replaced periodically because DOT 3 and 4 brake fluids are glycol-based and absorb moisture over time. This occurs whether a vehicle is driven 30,000 miles a year or just sits in a garage because fluid contamination is a function of time and humidity rather than mileage. Moisture enters the system past seals and through microscopic pores in hoses. It also enters every time the fluid reservoir is opened (a good reason not to do so unnecessarily). After only a year of service, DOT 3 fluid may contain as much as 2% water. After 18 months, the level of contamination can be as high as 3%. And after several years of service, it's not unusual to find brake fluid that has soaked up as much as 7 to 8% water. Many vehicles that are six, seven or eight years old have never had the brake fluid changed!

As the fluid soaks up moisture, it thickens and becomes less able to withstand heat and corrosion. The result is a significant drop in the fluid's boiling temperature, which may under the right conditions allow the fluid to boil in the calipers. Once brake fluid turns to vapor, the bubbles cause an increase in the distance the pedal must travel to apply the brakes. This condition should not be confused with "brake fade" that occurs when the brake linings get too hot as a result of prolonged braking. Brake fade requires greater and greater pedal effort to stop the vehicle while fluid boil increases pedal travel and makes the pedal feel soft or mushy.

The danger of fluid boil is greatest in front-wheel drive cars because of the higher operating temperatures that are generated in today's downsized front brakes, and because the hydraulic system is split diagonally. Semi-metallic linings compound the heat problem by conducting heat from the rotors to the calipers. If the fluid contains a lot of moisture and can't take the heat, it'll probably boil.

Caution: Use the type of brake fluid specified by the vehicle manufacturer. Though the owner's manuals for most domestic vehicles have no specific time or mileage recommendations for replacing brake fluid, recommending a change every two years for preventative maintenance is a good way to minimize the danger of fluid boil and internal corrosion in the brake system. At the very least, the fluid should always be replaced when the brakes are relined.

Some people say using silicone fluid eliminates moisture contamination problems. The premium-priced fluid, which is silicone based, does not absorb moisture and is theoretically a"lifetime" brake fluid. What's more, silicone fluid has a higher dry boiling point. But silicone brake fluid is very expensive (up to ten times as much as regular brake fluid!). It does not mix with non-silicone fluid, which means all the old fluid has to be removed if switching. And it is not recommended for any vehicle equipped with ABS because it contains a higher percentage of dissolved air that may cause foaming when the fluid is cycled rapidly. Other than that, it's great stuff.

In some cases, the brake warning light may remain on if the switch on the parking brake pedal or lever is misadjusted. Nothing is wrong with the brakes and a simple adjustment should fix the problem.

If the brake warning light comes on when you apply the brakes, or is on continuously, it means that hydraulic pressure has been lost in one side of the brake system or that the fluid level in the master cylinder is dangerously low (due to a leak somewhere in 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 the problem. But if there's a leak, 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 avoid driving the vehicle until the system can be inspected to determine what's wrong. 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 a leak that needs to be fixed.

Leaking brake fluid can also 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.

Brake Warning Light Circuit: The brake system is divided into two hydraulic circuits. On most rear-wheel drive vehicles, it is divided so one circuit applies the front brakes and the other applies the rear brakes. On front-wheel drive cars and minivans, the system is usually split diagonally. One circuit works the right front and left rear brake, and the other works the left front and right rear brake. This is done for safety purposes so if one circuit loses all its brake fluid and fails, the vehicle will still have one remaining circuit to apply two wheel brakes.

Usually located on the master cylinder or the brake lines exiting the master cylinder is a "pressure differential" switch. If either brake circuit develops a leak and loses its fluid, the difference in pressure when the brakes are applied will trip the differential switch causing the brake warning light to come on.

ABS Warning Light: On vehicles equipped with antilock brakes (ABS), a second warning light is provided to warn if a problem occurs within the ABS system. The ABS lamp comes on when the ignition is turned on for a bulb check, then goes 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 system.

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. Even so, 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 usually minor and the ABS system usually remains 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 special "scan tool" must be plugged into the vehicle's diagnostic connector to read out the fault code.

Diagnosing ABS problems requires a fair amount of knowledge and expertise (as well as special equipment in many applications), so this job is best left to a competent professional.

To check the booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit (the pedal will feel firmer and you won't hear any sounds from the booster). Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. If there's no change, the vacuum hose to the booster may be loose or blocked. If the vacuum hose is okay, the problem is in the booster and the booster needs to be replaced. If your brake booster has failed, your brakes will still work but will require increased pedal effort. The pedal will feel much harder and will take a lot more pressure to stop the vehicle. Driving with a bad booster can be dangerous because the vehicle may not be able to stop as quickly or in as short a distance. So don't delay. Have the problem diagnosed and repaired as soon as possible.

If your vehicle has an "integral" antilock brake system (where the ABS system is combined with the master cylinder assembly), power brake assist is provided by pump pressure stored in the ABS accumulator. (NOTE: If your vehicle has a "nonintegral" ABS system, it has a conventional vacuum brake booster.) If there's an ABS pump or accumulator failure, power assist will be lost and the ABS warning light should come on alerting you that a problem has occurred. Warning: If the ABS warning light is on, the ABS system is usually deactivated which means the ABS system can't prevent skidding when braking on wet or slick surfaces. You should have the system checked out and repaired as soon as possible.

Hydro-boost: Though not as common as vacuum booster power brake systems, some vehicles are equipped with Bendix "Hydro-Boost" power brakes. This system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide power assist. Pressure generated by the power steering pump is stored in an accumulator, which is then routed to the master cylinder by the Hydro-Boost unit when you step on the brakes. Problems can be caused by leaks inside the Hydro-Boost unit, by a worn power steering pump, slipping or broken pump drive belt, or hose connections. A simple way to test the Hydro-Boost system is to pump the brakes five or six times with the engine off to discharge the accumulator. Then press down hard on the pedal (about 40 lbs. of force) and start the engine. Like a vacuum booster, you should feel the pedal fall slightly when the engine starts, then rise. The leakdown of the accumulator can be checked by pumping the brakes several times while the engine is running, then shutting it off. Let the car sit for about an hour, then try the brakes without starting the engine. You should get 2 or 3 soft brake applications before it takes more effort to push the pedal.

Answer: While you are driving down a road, the car tends to drift to one side of the road. To keep the car straight, you must keep the steering wheel firmly in position. This problem creates a dangerous condition and normally appears gradually. Slight drifting to one side is often overlooked, and only severe pulling is noticed.
The probable causes are:
1. All of the car's tires do not have the same air pressure.
2. The wheels are out of alignment.
3. One brake is dragging or isn't releasing.
4. Loose steering parts and/or linkage.
5. The car's tires are not worn evenly.