June 2005 Articles & Features
By Karl Seyfert | June 2005
Even something as seemingly routine as a brake job can turn into a profit-destroying comeback if just a few small steps are overlooked or incorrectly performed.
Today's high-tech world of auto repair may occasionally make you wish for a return to a simpler, less complicated time. More than once, I've heard a frustrated tech exclaim “Just give me a brake job to do once in a while and I'd be happy.” Brake jobs and other basic maintenance like suspension service and tuneups were the bread & butter jobs that kept many shops in the black for generations. Indeed, entire service chains were established and flourished by catering exclusively to this type of work.
What's different now? Cars still have brake pads, shocks, struts and spark plugs. Yet hardly anyone even mentions these things anymore. Today, we seem consumed with concerns about diagnosing and repairing the electronic components that control an ever-increasing number of complicated vehicle systems. Learning how they work and how to repair them when they don't will continue to command a large percentage of our available brainpower.
If you had a brake job to do tomorrow, would you approach it with the same level of thoroughness and professionalism that you bring to other, more complicated repairs? Or would you rush through it distractedly, eager to return to the thorny diagnosis that's been occupying your thoughts for the past several hours? Too often, the second scenario leads to other unforeseen but perhaps completely avoidable problems.
This article isn't intended to show you how to do a complete brake job from start to finish. What it will do is highlight several of the small but very important steps that can be and often are overlooked in the rush to complete a “routine” brake job. The stuff we miss when we hurry more often than not results in an unwelcome and unprofitable comeback.
What follows is a short list of complaints that are sure to bring a customer back to your shop following a brake service. After we've identified the problems, we'll consider the steps that can be taken to cancel them out before the car leaves your shop.
Brake Squeal, Squeaks And Other Noises
One of the most annoying noises that any vehicle is capable of making is a high-pitched squeal or squeak when the brakes are applied. The squeal provides the driver with an audible cue that something isn't right, and may in fact be terribly wrong. And nothing will bring a customer back to your shop faster following a brake service, especially if the brakes didn't squeal before you worked on the car. An explanation that “they all do that” just isn't going to fly.
It's true that some brake noise is normal and, to a certain extent, unavoidable. But there are a number of small and reasonably simple things you can do to prevent abnormal brake noise.
First, examine the shims in place on the vehicle's original brake pads. They've already been subjected to a great deal of heat and may also be deterioriated due to exposure to the elements. If they look questionable, replace them. If you don't, you're inviting brake noise.
It may also be necessary to insulate the brake pad from the caliper with a spray or paste coating. The backs of the pads are coated prior to installation and this coating serves as a layer between the pad and caliper. Follow all manufacturer directions on where to apply and curing times. Many brake systems use a sandwich of materials (a shim) that's staked to the back of the pad. Pressure-sensitive adhesives may be used to secure the shim to the pad's backing plate. Clips may also be used to clamp the shim.
The type and quality of the brake pad material you install also can have an effect on brake noise complaints. Always use the recommended friction material when replacing brake pads and shoes. Using the incorrect type of material can cause noise and can affect the stopping ability of the vehicle.
The friction material used for pads and shoes can be identified by a code printed on the edge of the lining. This code identifies the manufacturer, the material and the cold and hot coefficients of friction. It does not indicate lining quality or hardness. When troubleshooting a brake problem, it's often a good idea to install new brake linings that match the OE friction specifications.
Carmakers may also specify different friction materials for the inboard and outboard pads, such as an organic pad on one side of the caliper and a semimetallic pad on the other. It's also common to see linings with a lower coefficient of friction on the rear brakes than on the front to minimize rear brake lockup, particularly on front-drive cars.
Noises other than squeals or squeaks, such as rattles or clunks, may have other causes, which will be covered in greater detail later.
Brake noise and brake pedal pulsation often go hand in hand because they may have similar, related causes. On disc brake-equipped vehicles, brake pedal pulsation is caused by rotor thickness variation. As the brake pads attempt to follow this variation, brake noise may also result. Pedal pulsation can also be caused by machining variations on brake drums.
Every brake rotor inspection should begin with a rotor thickness check. The nominal thickness is the rotor thickness when new. This specification can usually be found in the vehicle service manual. The machine-to thickness is the thinnest a rotor can be machined to and returned to service. The machine-to or minimum thickness specification provides enough rotor material for safe and reliable brake performance. If the rotor is too thin, has previously been resurfaced or has been damaged by worn-out pads, there's no point in wasting any more time on it.
Check the rotor thickness at eight points around the perimeter of the rotor. Never base your rotor thickness calculation on a single measurement taken at one spot on the rotor. Use a micrometer to measure rotor thickness, preferably one that's designed specifically for the job. A rotor mic features one flat anvil and one pointed anvil. The pointed anvil allows measurement of the real minimum thickness of a scored rotor. A mic with two flat anvils will contact only the shoulders of any grooves or scoring lines.
The discard thickness is the dimension at which the rotor must be replaced. If a rotor is at or below its discard thickness, brake performance will be compromised. Rotors worn below the discard dimension are also prone to warping and other damage. A rotor that's below the discard thickness should never be reinstalled.
The rotor friction surfaces must also be parallel to one another. The allowable tolerance is known as parallelism , or thickness variation. The variation might be the result of excess heat buildup that has warped the rotor, or the rotor may be contaminated by isolated thick spots caused by rust or corrosion buildup.
Lateral runout produces a rotor that appears to wobble from side to side as it rotates when viewed from its front edge. Runout can also cause brake pedal pulsation, steering wheel wobble and vibration during braking. Many brake complaints can be traced to runout problems.
Runout is defined as the amount of lateral (side-to-side) movement of the rotor as it rotates through 360°. The average allowable specification for late-model vehicles is between .001 and .003 in. This is the total indicated runout specification, as measured on the vehicle.
Use a dial indicator to check lateral runout with the rotor installed. Mount it securely to a stationary fixture. For on-car measuring, attach the fixture to the spindle or control arm. Locate the dial indicator's plunge tip about an inch inboard of the rotor edge, and zero the indicator.
If the rotors are out of spec and require machining, follow the appropriate resurfacing procedures, using an on-car or off-car brake lathe. As a finishing touch, a nondirectional finish should be applied to the newly refinished rotor.
Rotors should always be cleaned with soap and water after machining. A freshly resurfaced rotor is covered with metal particles that can become embedded in the brake pads. This could keep the new brake pads from transferring a layer of friction material onto the rotor, which can lead to noise, longer stops and increased pad or rotor wear.
The most effective cleaning method is a mild soap solution, with the rotor wiped dry with a clean rag or paper towels. Special soaps and cleaners will achieve a clean surface. Do not rely on brake cleaner and compressed air to do the job. Those who take the time to wash rotors before installation say it reduces noise comebacks.
Runout on new rotors should already be within accepted industry standards, with no additional machining required. You won't be able to improve on this by applying a cleanup cut before installing them. Removing as little as .0015 in. reduces rotor longevity. And if your lathe setup is incorrect, you may make the finish rougher or introduce a runout problem that wasn't there to begin with.
As with freshly resurfaced rotors, new rotors must be cleaned before installation. Most are treated with an anticorrosion coating that will contaminate the new brake pads. Remove it with brake cleaner or other solvent. If you drop the rotor in your parts cleaning tank, follow up with a soap and water bath to remove any solvent residue.
In the case of a hubless rotor, the hub itself may also have excessive runout, which is then transferred to the rotor. The contact area between the hub and hubless rotor hat may also be to blame. Contamination (rust or grit buildup) may create an uneven mounting surface that will cause the rotor to rotate off its intended axis. The hub-to-rotor mating surface must be free of rust, or runout-induced disc thickness variation can occur shortly after the brake job is completed. As little as .001 in. of rust at the outside edge of the hub will translate into .002 to .004 in. of runout when measured at the outside edge of the rotor.
The method used to clean the hub depends on the severity of the rust buildup. The hub's mating surface can be a difficult surface to clean due to the wheel studs. The area between the wheel stud and hub-centering flange is the most difficult area to gain access to. If there's mild rust buildup on the hub, use an angle grinder equipped with an abrasive disc to clean the majority of the surface area. Abrasive blasting can be used for heavy rust removal. Short of removing the hub from the vehicle, specialized equipment is available to blast the hub while it's still on the vehicle.
If runout is still above the .002-in. range after cleaning, try indexing the rotor to reduce runout as much as possible. Place chalk marks on a stud and at the adjacent area on the rotor hat. Now relocate the rotor clockwise to the next stud position before repeating your runout measurement. Repositioning the rotor may minimize the total assembled runout caused by minor deviations on the machined surfaces of both the underside of the rotor hat and the hub flange. If you encounter a large number of new rotors with more than .003 in. of runout that can't be traced to any other cause, it may be time to shop for a new rotor supplier.
Rotor runout can be measured independent of the hub by removing it from the vehicle and mounting it on an off-car brake lathe. Likewise, hub runout can be measured without the rotor in place, to remove as many variables as possible during a complicated runout diagnosis. If the hub flange or rotor is causing the runout problem, you'll be able to isolate either in this way. Dedicated compensation plates that are installed between the hub and rotor are also available to reduce the runout to an acceptable level that will be imperceptible to the driver. The compensating plate goes between behind the rotor, with the thickest part 180° opposite of maximum rotor runout.
Another potential cause of excessive runout (one that's common with thin-hat hubless rotors) is improper wheel installation procedures. Many perfectly machined, cleaned and indexed rotors have been ruined by the last step in the procedure�tightening the wheel lugs. Ideally, this procedure should be accomplished using a properly calibrated torque wrench. An impact wrench used at full throttle will quickly distort a hubless rotor, and may also damage the wheel lugs. If you're not willing to use a torque wrench, at least invest in a set of torque-limiting impact sockets.
Never use lubricants or penetrating fluids on wheel studs, nuts or mounting surfaces. They should be clean and dry. A thin layer of lube may be applied between the inner surface of the rotor and the hub to slow down corrosion.
The brakes may apply smoothly, with no evidence of noise or pulsation, yet the steering wheel may attempt to turn to the left or right when the brakes are applied. The pull is usually only to one side or the other. In rare cases, the vehicle may appear to wander in either direction when the brakes are applied. A pull caused by the front brakes is usually severe, and tugs on the steering wheel. Rear brake pulls cause the car to drift into the next lane and make the car feel unstable, while having little effect on the steering wheel.
A stuck caliper is the first thing many techs suspect when a brake pull complaint affecting steering is reported. A stuck caliper can cause a vehicle to pull to one side or the other, even when the brakes are not applied. After a test drive, compare the heat coming from each wheel using a noncontact infrared thermometer. If one wheel is obviously hotter, suspect a brake problem, not a suspension or steering problem.
But what do you do after the suspected caliper has been replaced and the problem persists? Have you checked tire inflation pressure and inspected for mismatched tire sizes? These can cause the vehicle to pull to one side at all times, but the pull may become more noticeable during braking. A bad tire may also cause a pull. This can be quickly determined by swapping tires from side to side. If the pull follows the tire, you've found your problem.
A damaged brake hose may also produce a brake pull. A hose may look okay on the outside, but the inner liner may be damaged or collapsed, possibly causing a restriction. The restriction can reduce braking power, or may cause brake drag if it acts as a check valve.
A restriction in the hose will cause the vehicle to pull to the side with the good brake hose upon initial application of the brakes. The pull may go away after a second or two because the side with the collapsed hose will apply later than the one with the good hose. It takes longer to get enough fluid volume into the caliper to move the brake pads against the rotor. The pull may be extreme during initial pedal application, but go away as the fluid slowly moves the piston.
A damaged hose also may act as a check valve, allowing fluid to flow into the caliper, but preventing it from returning to the master cylinder when the pedal is released. The symptoms of this type of damaged hose can mimic a stuck caliper piston, as the caliper will fail to fully release after the pedal has been released.
If one brake is hotter than its mate after a road test, don't be too quick to condemn it. If the left front brake hose is damaged and the caliper is not applying properly, the right front caliper will be hotter because it's doing more than its fair share of the work. The brake that's not doing much work will be cooler and the vehicle will pull in the direction of the good caliper.
Put the vehicle on a lift, then compare how easily the dragging wheel spins compared to the opposite side. If it's difficult to turn by hand, loosen the bleeder screw. If the wheel turns freely after pressure in the caliper is released, the caliper is okay. If only one wheel is affected, a damaged brake hose is most likely to blame.
To make certain the problem isn't located further upstream, reapply and release the brakes, then loosen the brake hose at the fitting where it joins the steel line. If the problem goes away, the proportioning valve, master cylinder or ABS system must be inspected.
If loosening the bleeder screw does not allow the wheel to turn freely, the piston is most likely seized in the caliper bore, or perhaps the mounting hardware is preventing the caliper from releasing the outboard pad (more on this later). With the bleeder screw open, the piston should push easily into the bore. If it sticks or if there's obvious damage to the caliper piston, seal or bleeder, replace or rebuild the caliper.
A pair of brake pressure gauges can be used to diagnose a brake pull due to uneven hydraulic pressure. The gauge's load cell is installed in place of the inner brake pad, after the caliper is moved free of the rotor. With two gauges installed, the actual force applied to the load cell can be checked, making it easy to compare pressures side to side. This can find mechanical caliper problems as well as hydraulic faults.
Have an assistant depress the brake pedal while you watch the gauges. Observe the maximum pressure to each wheel, as well as the time it takes to reach that pressure. A restricted brake hose will cause the pressure to build more slowly than a free-flowing hose. A restriction that creates a one-way check valve will also cause the pressure to drop slowly when the brake is released. A dented metal brake line can cause the same slow response as a defective brake hose. Air in one brake line can also reduce the available pressure and cause brake pull.
An additional potential cause of brake pull is mismatched friction material. Mismatched linings can occur when a do-it-yourselfer replaces pads on just one side. The same thing can occur when a professional installs a single “loaded” caliper. The vehicle now has mismatched calipers (one old and one new) and mismatched friction material. There's a big difference in the coefficient of friction between organic lining and semimetallic lining, for example. Loaded calipers should be replaced in pairs to assure matching friction material and caliper performance.
Brake linings that have become con-taminated with grease, oil, brake fluid, etc., can also be the source of a pull. Cleaning will not rid the lining of con-tamination; it absorbs the contamination like a sponge. Replace the linings after the source of the contamination has been determined and repaired.
Premature or Uneven Pad Wear
It may be weeks or months before a customer returns with a brake complaint. This time it may not be due to a noise or pulling problem. Instead, the complaint may be that the brakes have worn out prematurely or have worn unevenly. Perhaps the brakes on the left front still have many miles of wear left, while the pads on the right are worn to the backing plates. Or, more puzzling, the outer pad on the right side may be completely worn out, while the inner pad looks nearly new. What gives?
The devil is in the details. Overlooking the smallest parts can sow seeds for problems that may take months to sprout. It begins with the brake hardware. What condition is it in? If the caliper hardware is corroded or seized, repair or replace it as necessary. Many premium brake pad sets include a new set of antirattle clips, springs and pad shims. All of these parts are designed to aid proper brake application and to quell noise and vibration. But they can't do their job if you don't install them.
Slapping on a new set of pads without servicing the hardware will produce predictable results. The tip-off will be the relative thickness of the inner and outer pads. If the outer pad has worn more than the inner pad, the caliper hardware is to blame. If both pads are worn out, a stuck caliper piston is more likely to blame. Remove the caliper from the knuckle and check for seizing or binding of the caliper mounting hardware.
Individual pads may also wear unevenly, ending up thinner at one end than the other. This problem is caused by damaged or corroded brake pad retaining springs and clips. The corrosion exerts a firm grip on the pads, preventing them from moving in and out as the brakes are applied and released.
In extreme cases, corrosion may develop a hold that's strong enough to completely resist the brake system's hydraulic pressure. When the brakes are applied, the pad or pads don't even move. The driver may complain of reduced braking efficiency or pulling because the brakes at one or more wheel are not doing their fair share of the work.
Heating and cooling cycles can weaken springs and antirattle clips. Weak hardware can result in excessive caliper/pad movement or binding, causing noise and other related problems. This can lead to uneven and premature pad wear, rotor warpage and pulling. Make sure all of the brake system's shims and antirattle clips are in good condition and will last as long as the new pads you're installing. If anything looks worn or otherwise suspect, replace it.
Don't overlook the brake system's dust shields, grommets and bushings. If the soft parts still look serviceable, clean out all the old lubricant and replace with new lubricant. Make certain the lubricant you use is formulated for brake use. An approved brake lubricant should be able to handle 400°F, and should contain no petroleum distillates or any other substance detrimental to soft parts.
Most floating brake calipers use a rubber or plastic insulator or shim around the caliper mounting bolts. The high-temperature brake environment can cause this sleeve to lose its resiliency or become flattened into an oval shape, allowing excessive caliper movement. This can cause rattles and thumps when the brakes are not applied. The affected brake may also apply unevenly, resulting in a pull. Loose bolts or worn hardware can cause the caliper to cock when applied.
Don't forget the wheel bearings. A loose wheel bearing causes excessive play or looseness at the wheel hub. The excessive play may actually push the pads away from the rotor, causing a low brake pedal when it's first applied. A worn or misadjusted wheel bearing causes the brake rotor to be out of alignment with the caliper and brake pads. A caliper may also be out of alignment with the rotor if the caliper mounting points have been bent due to collision or other damage. A crooked caliper will inevitably result in uneven braking and pad wear.
Any extra time invested in doing a complete brake job will be rewarded with fewer comebacks and more satisfied customers.