Diagnosing and correcting brake life and performance issues
When a vehicle’s brake pads and/or rotors wear faster than expected, a host of variables need to be considered, including materials, prior installation methods, driver abuse, etc. In this article, we’ll discuss potential issues and offer suggestions for checking and correcting customer concerns regarding brake life and performance.
Pad and rotor break-in
As Rick Kearns, Curriculum Training manager at the Federal-Mogul Technical Education Center in St. Louis noted, break-in is probably the top reason for premature wear. The need for proper break-in has become more important in recent years due to the use of lighter weight materials, rotor metallurgy and pads materials. The two mating surfaces (pad and rotor) need to transfer material (pad to rotor and rotor to pad) in order to seat together properly. Kerns recommends the tried ‘n’ true “30/30/30” method, involving 30 stops, each from zero to 30 mph, with a 30 second cool-down time between each stop.
As the pads mate with the rotor disc surface, and depending on the type of pad formula, a small amount of friction material is transferred to the disc surface, which increases braking efficiency.
Clean, clean and clean!
Proper washing of a brake rotor is all too often ignored. Any brake rotor, new or used, must be thoroughly cleaned prior to installation, in order to remove any and all contaminates.
New rotors are treated to a temporary anti-rust coating to prevent surface corrosion during packing, shipping and storage. This might involve a simple oil or a waxy-type coating. Regardless, this MUST be removed before installing the rotors. The common approach is to spray the disc surfaces with a brake cleaning solvent or a quick wipe-down with a rag and solvent (brake cleaner, lacquer thinner, etc.). While a solvent cleaning is done with good intentions, rotors should be final-washed in HOT water and an effective detergent. My favorite is Dawn dish washing liquid and hot water.
Once cleaned, the rotor should be handled while wearing clean latex gloves. During rotor, caliper and pad installation, do your best to avoid contaminating the rotor disc surfaces. Understandably, this is difficult to avoid, but if/when dirty or greasy fingerprints appear on the surfaces during installation, clean those areas again with a spray-on brake cleaner, preferably followed by a spot-scrubbing with hot soapy water and rinsing. Any contaminants remaining on the disc surfaces will quickly transfer to the pads, reducing the coefficient of friction that would be available if the rotor were properly cleaned at the outset.
In addition to rotor cleanliness, pay attention to the caliper mounting. Make sure that the caliper pins, guideplate and bridge are clean. That means eliminating all rust buildup on the bridge plate. Remove the bridge, wire brush any rust buildup, bead blast it and apply a protective coating (SEM etching primer and urethane paint for example). Rust tends to build up under the guideplate, which can easily affect caliper fit and movement. An example of common caliper hang-up issues involves some GM SUV models equipped with four-wheel disc brakes, where rust tends to build up below the guide plate causing the pads to lock in place and wear prematurely.
While rotor runout specifications may differ depending on make, model and year of the vehicle, generally speaking any rotor with more than 0.002 – 0.005 inch lateral runout should be either resurfaced or replaced. With the wheel removed, secure the rotor to the hub with at least three fasteners, torqued to 40 ft.-lbs. Mount a dial indicator (anchored rigidly to a non-moving surface) with the indicator gauge probe contacting the rotor disc surface at a 90-degree angle, about one inch from the outer edge. Preload the indicator about 0.050 inch, then zero the gauge. Slowly rotate the rotor and observe the gauge (rotate a full 360 degrees several times). Using a marker, mark the point of highest lateral runout and compare your reading to the manufacturer specification.
Remove the rotor, and clean the mating surfaces on the rotor and hub. Place the dial indicator on the hub itself and perform a runout check, and compare your reading to the factory specification. Check wheel bearing end-play (and replace or adjust as needed).
Install the rotor (with mating surfaces cleaned and free of rust/residue). Re-check rotor lateral runout. If rotor runout is still excessive, remove the rotor and reinstall in the next clockwise location (moving to the next fastener locations). Re-check runout.
Repeat the clockwise indexing of the rotor throughout a 360-degree sweep. In some cases, a stack-up of runout tolerances between the rotor and hub may improve (or worsen) the measured runout at the rotor. If excessive runout is observed after testing at different clock locations, the rotor must either be resurfaced or replaced.
Inspect the rotor for parallelism. Using a brake rotor micrometer, measure rotor thickness, one inch from the outer edge, at six to eight clock position points. Compare your findings to the specification. Generally speaking, maximum allowable parallelism should be about 0.0005 inch.
Resurfacing today’s rotors can sometimes present a challenge, as thinner rotor hats may not allow accurate placement on a stationary resurfacing machine. The best method is to use an on-car resurfacing machine. This allows you to resurface (true) the disc while it’s mounted on the vehicle, which includes any variables that can influence runout, such as runout on the hub. Resurfacing the disc on the vehicle provides a more accurate truing as it rotates in its installed position.
Runout-correction shims are available for certain vehicle applications, but if proper runout-checking (and corrective measures) are followed, there should be no need to resort to the use of shims.
Brake rotors that feature excessive runout will not only produce a bouncing brake pedal, but can also contribute to premature and uneven pad wear. Also, in cases where pedal bounce is felt by the driver, he or she may have a tendency to apply additional pedal pressure, which simply adds to pad damage, possibly overstressing the backing plates to the point where they begin to distort/bend, which results in minimized pad contact with the disc and potential cracking of the friction material.
Today’s brake rotors generally feature a non-directional surface finish. If the rotor needs only a minimal resurfacing, block-sanding (while the rotor is rotated) can be done starting with 120-grit, followed by 150-grit paper, using moderate pressure. This will aid in averaging-out the peaks and valleys in the disc surface. Care is required, as excessive pressure can create waviness of the surface.
An unevenly worn or distorted rotor disc will obviously create a brake pedal pulsation, as high and low contact areas pass through the pads during brake application. While rotors of old were commonly made of robust materials and featured heftier thickness in the hat area, today’s rotors have become lighter (a result of the OEs chasing more rigid fuel economy standards). Thin-hat rotors are very susceptible to distortion, usually the result of improper wheel fastener tightening. That’s why it’s important to evenly tighten all wheel fasteners (in a crisscross pattern), with identical torque value applied to each fastener.
The use of a calibrated torque wrench is the recommended way of tightening wheel fasteners. An alternative involves the use of torque “sticks” that enter an elastic range upon reaching a specified torque range, but these are easy to misuse if an installer repeatedly hits his air wrench at the same fastener. A practical and viable alternative is to use one of the new cordless impact wrenches that employ a digital clutch for setting specific torque values. But if you’re not in a rush, a hand-operated torque wrench is recommended.
While we’re on the subject of torque wrenches, note that torque wrenches that experience a high rate of use will eventually lose their calibration. It’s a very good idea to have all of your micrometer “click” type torque wrenches sent out for recalibration on a regular basis (at least once per year). The cost of this service is fairly inexpensive (usually ranging around $25 or so), and well worth it.
Try to cycle your shop’s torque wrenches throughout the year, one by one, so that you always have one to use on a daily basis. The calibration service shop will usually place a label on the tool indicating the date it was serviced.
Measuring rotor disc thickness allows you to compare existing thickness with the factory specification and tolerance range. Nominal thickness represents the acceptable “new or fully serviceable” thickness, while the discard thickness indicates the reduced thickness at which point the rotor is no longer viable and must be replaced. For example, a rotor may be marked with a nominal thickness of 1.102 inch and a discard thickness of 0.984 inch. Once that rotor gets down to (or below) the discard thickness of 0.984 inch, it must be replaced.
When measuring rotor thickness, the use of a micrometer is needed. Rather than using a conventional micrometer that features flat-surface anvils at each end, a dedicated brake rotor thickness micrometer or caliper can provide a slightly more precise measurement, featuring a flat anvil on one end and a pointed anvil/probe at the opposite end. This helps to eliminate discrepancies when using a flat/flat micrometer if the tool is held slightly cocked on the rotor. Never use a common incremented or digital caliper to measure rotor thickness, since the contact areas won’t allow measuring of the wear surface area. You need a measurement tool that features a small contact footprint and that will avoid the thicker (and rusted) outer edge area.
When measuring rotor thickness, stay away from the outer edge, where the pads don’t make contact and where rust buildup is found. The measurement should be taken approximately one-half inch to three-fourths inch from the edge (refer to the vehicle service manual for the recommended location).
Caliper and pad hardware
Brake pad-to-caliper hardware should always be replaced when installing new pads. If the pads don’t include the required clips, order them separately. Locating and spring clips gradually lose their tension via brake heat exposure, and sliding surfaces can become galled and rough. Clips that allow excess pad movement can allow the pads to cock or flutter during use, which can lead to excess brake noise and uneven pad wear. Pad hardware is cheap. There’s no excuse for ignoring this.
Where vehicle design features sliding calipers, the slider pins can become corroded and rough, preventing proper lateral movement of the caliper body when brakes are applied and released. Pins and their boots are inexpensive but critical for proper sliding caliper operation. When in doubt, replace. Sticking sliders can easily result in placing excess force at one pad while minimizing frictional force at the opposite pad.
Lubricate all contact points where the pad backing plate meets the caliper bridge, using a high-temp moly or hi-temp silicone grease (NAPA’s Syl-Glide is a good example).
Sticking calipers and imploded hoses
Calipers that stick or flexible brake hoses that “implode” (collapse) can easily contribute to uneven pad wear. If the piston(s) in the caliper cannot move freely, sticking in various positions or in the extended position will place excess pressure on inboard, outboard or both pads. Degraded brake hoses that tend to implode when the brake pedal is released can cause a restriction in the hose, preventing full release of the pads from the rotor disc.
If you suspect sticking pistons in a caliper, inspect closely for worn, degraded or burnt piston seals.
None of us have X-ray vision, so it’s difficult to see what’s going on inside the caliper piston bores. If a sticking caliper is suspected, remove the caliper, place it in a bench vise and insert a hefty shop rag inside the pad cavity and carefully apply shop air to the bleeder to push the piston(s) out. You may discover galled or rusty pistons and damaged seals. Never assume that what you can’t see can’t be the problem.
Hose implosion may be temporary, gradually releasing pressure after the brake pedal has been released. This will cause residual pressure in the caliper, preventing the pads from fully releasing from the disc. The result is obvious: If there is a pressure difference (wheel to wheel on the same axle), of if pressure does not release fully causing pad drag, premature pad wear is inevitable.
One very handy specialty tool that I’ve discovered in Innovative Products’ Disc Brake System Analyzer, which features a pair of mechanical pressure gauges. Using this kit avoids the need to open any hydraulic lines for testing purposes. Each gauge features a pressure contact block and a pressure gauge. By removing the caliper’s inboard pad and placing the tester block in its place (between the disc and caliper piston), brake system pressure can be observed on both split diagonal and split front-rear systems.
Each tester block features a round pressure “pad” (this round pad contacts the piston). As the brake pedal is depressed, the pressure gauge red needle reads and holds the peak pressure for subsequent study. The gauge also provides an indication of a sticking caliper or imploded hose (if the black needle doesn’t return to zero upon releasing the pedal).
We’ll provide a full in-depth review of this tool in-use in an upcoming issue.
Even though experienced brake technicians are adept at surveying pad thickness and making a go-no-go decision based on visual inspection, pad thickness should be measured and compared to OE specifications. Measuring pad thickness normally requires pad removal, simply from an access standpoint. To make life a tad easier, OTC offers a cute little pen-style pad thickness gauge that can be used without pad or caliper removal. The tool is placed so that the tip contacts the rotor disc surface and a sliding hook contacts the inboard face of the pad backing plate. An incremented gauge (similar to a pencil type air pressure gauge) offers 1 mm increments as well as color coding. With brakes applied by a co-worker (to make sure that the pads are in full contact with the disc), insert the tool, rotate the knurled ring on the measuring barrel and read the distance. If the reading is 8 mm or more (green zone), pad thickness is OK. If 3 mm to 8 mm (yellow zone), pad replacement is suggested for the near future. If 0 mm to 3 mm (red zone), replacement is immediately mandatory.
Of course, it’s obvious that one potential cause of premature pad wear can be linked to the driving style. As we all know, some customers have the habit of “riding” the brakes when they should keep their left foot off of the pedal. Unfortunately, there’s nothing that we can do to prevent this, short of trying to educate the customer.
Brake pad construction
Information on pad makeup was presented in a previous issue of Auto Service Professional (see “Brake pad and rotor technology” in the January/February 2014 issue) available in print or on our website at www.autoserviceprofessional.com.
Quite often, premature brake pad wear may be the result of a misapplication for the intended use. Especially for heavy vehicles, towing applications or spirited performance, pad formulas that are intended for “normal” street use can wear faster than the vehicle owner expects.
Brake pad friction material is designed for a make/model vehicle’s gross vehicle weight. It’s best to replace pads using the original formulation. With that said, we can make a few generalizations with regard to pad type. Semi-metallic pads tend to withstand heat and offer better cold stopping as compared to ceramic pads. However, ceramic pads generally produce less pad dusting and better noise reduction. Ceramic pad stopping performance is enhanced with the use of larger contact area (larger pads and larger rotors). Carbon-based metallic pads may be a good choice for vehicles that are routinely driven in hilly/mountainous terrain. Today’s pad makers offer a wide selection of pad formulations both for specific vehicle applications as well as specific driving environments.
Physical pad design
Depending on the vehicle or intended use application, pads may feature one or more slots. These slots in the friction material pad help to clean dust out of the rotor-to-pad contact area, and provide thermal expansion of the friction material segments, which aid in preventing pad material stress cracking. Slots also help to minimize the effects of pad gassing. Pads may feature chamfers at the leading and exit edges of the pad. Leading edge chamfers aid in increasing the full contact footprint of the pad to the rotor, providing a less-abrupt attack point as the pad meets the rotating disc. As gassing increases, the pad wants to lift off of the rotor surface, resulting in pad bounce, which can create undesirable harmonics. Chamfering is often used to eliminate or “tune” certain noise frequencies as well. Both slotting and chamfering help to minimize the gassing effect.
According to a research study by NUCAP Industries Inc., an alarming number of brake pad sets sold in the aftermarket don’t meet OEM specifications/pad fit. NUCAP’s study reports pads that are manufactured in the U.S. meet OE specs 100% of the time.
Dandy dust shield
And finally, here’s a tech tip: Many GM truck/SUV vehicles are known for badly rusted rear dust shields. Normally, replacement involves axle disassembly. A handy fix is offered by Dorman Products Inc. via a two-piece split-shield design that eliminates extensive disassembly work. P/N 924-213 covers 2000, 2002-2006 Cadillac Escalade, 1999-2010 Chevy Yukon, Tahoe, Suburban, Avalanche, Silverado and Express, and 1999-2010 GMC Savana and Sierra vehicles. ●
Want more technical articles? To read the entire May/June 2014 issue of Auto Service Professional, see our digital version by clicking here.