Brakes 101 ...

I found this article a few years back.
Covers everything about brake pads, rotors, Pulsing brakes, turning rotors, lugnut torque, etc etc.

It's a great read , though it is lengthy :thumbsup:

One factor that's still being neglected, however, is proper lining break-in. Some techs believe the right way to seat linings is to really stand on the brakes a few times. That's an anachronistic idea left over from the days when linings were supplied. "green." Panic stops would indeed get that friction material hot enough to cure it. But you don't get uncooked pads and shoes from manufacturers anymore, so this whole idea belongs to a bygone era, a time of ignition points and bins-ply tires.

Several authorities say botching break in is the primary cause of noise and hard pedal complaints. The ideal way to start new linings off is to make 30 slow stops (spaced two minutes apart) from about 30 mph using light to moderate pressure. Obviously, no working tech is going to take that much time. But you'd be foolish not to make at least 10 moderate stops at 30-second intervals (you should be able to feel the action smooth out), then caution your patron to avoid heavy braking for the first 200 miles.

If you don't open the bleeder before you push the caliper piston back, you'll force debris and watery fluid up into the ABS ...
...but many experts say you should also clamp off the line. Use the proper tool, please.
Be careful while working around wheel speed sensors, and make sure there are no metal particles stuck to the magnet.

Plugging the master cylinder's outlets solid is a great diagnostic trick. It's also the preferred method of bench-bleeding today.

Is that rear disc caliper's adjustment mechanism seized, or is the car's owner just not using the parking brake?
Replacing drum brake self-adjustment hardware will help insure against a low-pedal complaint and premature front lining wear.

Sure, DTV is what causes pulsation, but run-out is the reason discs wear that way.
Short of replacing rotors, the best long-term cure for brake shudder is the use of an on-car lathe.

Brake Job Fine Points.
The subtleties that make the differences between the pad hanger and a craftsman -- and between disappointed customers and loyal patrons
These days, all of your professional journals burn up a great deal of editorial space on driveability issues. As far as dollar volume for the typical shop is concerned, however, brake work makes up a much higher percentage. So, we at Motor Service try to give you more and better brake coverage than anybody else. Keep reading and you'll avoid some common mistakes, make extra money from truly-justifiable add-ons, and maybe even become a hero to your customers.

Contaminant containment
We still see people push caliper pistons back to make room for new pads without first opening the bleeder. Sure, you could get away with that in the old days and maybe avoid having to bleed the system, but now there's a good chance you'll cause ABS problems by pumping sediment (a combination of rust and the ashy residue of burned glycol) ad watery fluid up into all those tiny passages, valves, and pistons. As one brake parts company training, manager says, "If you get sediment up into a 4WAL, for example, and it jams a piston, you'll have to replace the unit, which is $1,150 dealer cost. Even a reman is $850." Of course, it's not always so bad. With some units you can use a scan tool to put the system into the self-utilization cycle and pump itself clean, but why take the risk?
Lots of authorities say that just cracking the bleeder isn't good enough, citing the fact that contaminants naturally accumulate at the bottom of the cylinder where the hose comes in. They recommend that you also clamp the hose using a suitable smooth-jawed tool, and we agree. If you're afraid that hose looks too whipped to survive clamping, you should be replacing it anyway.
Flush 'em out

This brings up fluid flushing, something the domestic car companies are just getting around to including on maintenance schedules (most foreign auto makers have done that for many years). Not only are corrosion and contamination of intricate and expensive ABS hardware to be strenuously avoided, but the same goes for a typical four-wheel disc system, wherein the parking brake/self-adjustment mechanism is immersed. Another factor is the high operating temperatures encountered with semi-mets and FWD, which makes maintaining a high boiling point critical to safety. Fluid changes are cheap insurance. Depending on whom you talk to, recommended intervals range from one to three years. Our opinion? Two, max.
Since this is a relatively new service item that the average motorist has never heard of, you'll need a good approach if you're going to convince him to have it done and thus save him big bucks in the long run. First, pull the owner's manual out of the glove compartment and show him where it appears on the service interval chart. Explain how disastrously expensive internal corrosion can be, and mention that in Europe people do this almost as religiously as they change their oil. Also, you might want to keep those "Wet Check" brake fluid test strips handy, or you could invest in an electronic moisture detector.
Speaking of ABS, another thing to remember is that it's asking for trouble to start a brake job on a car so equipped if you don't have the right scan tool. With Delco VI and Bendix systems, to cite two prominent examples, if you do something that causes the anti-lock warning light to come on, you won't be able to turn it out. Also, it's easy to damage those delicate wheel speed sensors while doing brake work, or to cause metal particles to attach themselves to the magnet. Either is likely to put the system into default, or make it go permanently into anti-lock mode (called "false modulation"), and turn on the warning lamp.

The most horrifying hydraulic-system error we can think of is letting a dangerously rusted line or chafed hose pass inspection. Call us paranoid about this subject if you want, but we had two total brake failures arrive at our shop within a few months of each other, one involving an accident, just because nobody had ever taker a look at those lines, and we once almost took an icy dip in Lake Michigan when a front hose blew. These are not the kinds of comebacks you want on your conscience.

Poor pedal
Pedal-feel problems are at the top of the, list of common hydraulic shortcomings. Hotline experts tell us they get more calls on this subject than on anything else, so don't send that car out even though you're not satisfied with how it feels. Find the problem and fix it.
We've heard that air in the system is the culprit in 90% of low- and spongy-pedal complaints, yet the master cylinder gets wrongfully blamed much of the time. In fact, manufacturers have told us that 75% of all the masters they get back under warranty are really okay.
Before you do something embarrassing, verify that the master cylinder is okay by removing the lines, screwing brass or plastic plugs (either ISO or double flare) into the outlets, then applying the brakes. If the pedal's high and hard now the master's properly bled and its seals are okay because the pedal would sink gradually if it were bypassing.

Continue in this process of elimination by capping lines or clamping hoses (again, use a suitable rounded jaw tool please, not sharp-toothed Vise Grips) to isolate the wheels. Releasing one at a time should locate the problem.
Faulty bleeding procedures can obviously leave air in the system and cause pedal complaints. For example on Teves Mark II ABS systems, you can't get fluid to the rear brakes unless you turn the key on. Also, plugging the master cylinders outlets mentioned above has become the preferred means of bench bleeding. Just keep stroking until no more bubbles appear at the compensating ports and you can only move the piston maybe 1/16 in.

Take up space
Don't think of low pedal as strictly a hydraulic problem. With most modern rear disc set-ups, the parking brake is often overlooked as a cause of excessive mechanical movement between linings and rotor. If the customer never engages this safety device, as many do not, adjustment simply won't occur, and that means a low pedal. Another impediment to adjustment is corrosion and contamination of the piston, cylinder, and screw mechanism.

The first step in fixing this situation is to talk that motorist into using the parking brake. Then, go hands-on and overhaul or replace the calipers if they're not just right.
Don't forget drum brakes. We're surprised when we DON'T see seized star wheel screws and otherwise inoperative self-adjusters, so insure against a comeback by replacing that hardware. Weak drums, either from machining to or even beyond the limit, or from heavy rust, can also cause a pedal problem.

There's another factor that's usually not recognized:
Careful drivers who never stop hard enough in reverse to ratchet that mechanism. The answer to this is to do an initial adjustment, then tell the customer that an inspection and shoe-to-drum clearance setting should be done at every oil change.

With multi piston rigidly-mounted calipers, axle end play is something to watch out for. If there s more than about .004 in., a low pedal situation is apt to obtain because each time the axle moves in or out it pushes those pistons back.

Premature demise
Right here we should mention that anything that interferes with the rear brakes doing their fair share of the work won't only cause a low pedal, but will also greatly accelerate front pad wear. Let's put it this way: Whenever we're presented with a vehicle that's started to eat front linings, the first thing we check is the back brakes.
Something else that can wipe out linings prematurely is dragging brakes. For instance, we had a FWD Chevy in our ad fried pads and a badly grooved rotor on one side. The hose was plugged so that it allowed pressure to gradually build up in the caliper, and the piston couldn't retract. Another possible cause of drag is over-filling the master reservoir, which car apply the brakes as the fluid expands. By the way, excessive outboard pad wear usually means that somebody neglected to lube that caliper hardware with the proper silicone grease.

Pedal pulsation plague
A pulsating pedal is certainly one of the most prevalent brake problems today. We've often heard the statement, "Rotor thickness variation, not runout, causes pedal pulsation," but this is a half-truth that causes consternation. The direct and final cause of pulsation is indeed DTV (Disc Thickness Variation), which can also be seen as a lack of parallelsim between the two sides of a rotor. But wobble is the primary reason discs wear unevenly -- they hit those abrasive pads in one spot on each side every revolution of the wheel, and the contact areas end up thinner than the rest of the rotor. This is practically guaranteed to result in a complaint as the fat places knock the pads and piston back, and that little column of fluid transmits this movement to the pedal. One on-car lathe manufacturer claims that, typically, 002 in. of runout with zero-clearance bearings will cause about .0004 in. of DTV in 3,000 to 5,000 miles.

Some people want thickness variation to be held to .0002 in., while others say it'll take .0004 to generate a complaint. Either way, you've got to take your time checking for discrepancies this small.
First of course, you've got to determine whether the pulses are coming from the front or the rear (or both). Commonly, DTV in the front will cause the steering wheel to shudder on light brake applications at low speeds. You'll feel the side-to-side movement if you hold the wheel with just a few fingers. To ascertain if the rears are at fault, find an uncrowded street, coast in neutral at about 20-25 mph, and apply the parking brake gradually. Out-of-round drums will typically produce a heavy bumping and hitching during this test.

Get the car safely up in the air, but don't pull the wheels yet. Instead, measure runout on the inside of the rotor with the wheel installed and the lugs torqued, if possible. This will tip you off to real-world runout. Finally, measure thickness at eight evenly-spaced points around the disc. By the way, you'll need both a 0-1 in. and a 1-12 in. micrometer to cover cars and light trucks.

Avoid ABS problems during routine brake service.
Motor Age; 2/1/2003; Buckley, Jay M.
Antilock braking systems (ABS) have had more of an impact on brake servicing than may seem obvious. And although most jobs are routine and your customer leaves satisfied, on occasion somebody will come back in to your shop with a complaint.

The most successful technicians have mastered the art of quick -- but complete -- installations. Most comebacks aren't the result of something intentionally done wrong, but may be caused by something being overlooked.
I get the following call at least once a day: "I just did a routine front brake reline and now the car's:
* ABS light is on.
* Brake pedal is low.
* Brakes are dragging.
* Pulling to the left/right."

These problems can be avoided by adopting some simple and very effective practices when performing brake service on an ABS-equipped vehicle. The number one cause on these problems is very simple, and to prevent it from happening, technicians need to modify the way they are currently doing a brake job.

It is extremely important to open the bleeder screw when compressing caliper pistons or wheel cylinders during routine brake maintenance. When a caliper piston is pushed in, brake fluid gets forced backward, up into the system. Since the caliper is the lowest point in the system, dirt and corrosion naturally accumulate there. When this grime and dirt finds its way into the HCU portion of the ABS system, it can cause valves to stick. This can lead to: a vehicle pulling one way or another; accumelators sticking open, which leads to low pedal; compensator ports pluging, which leads to dragging brakes; and of course, it can make the ABS light come on due to all of the above.

If technicians get in the habit of opening the bleeder when they compress the caliper piston, as well as selling customers a brake fluid flush, these problems will more than likely be prevented.

Another problem that concerns installers is when the ABS applies itself at low speed. Wheel speed sensors are magnetic. They can collect large amounts of debris from the road. Freshly turned rotors also can leave metal filings on the sensors, especially if an on-car brake lathe is used. The Honda Motor Go. actually recommends removing the sensors during rotor machining. We suggest cleaning the wheel speed sensors and tone rings during a brake job. This can help prevent false signals that can actuate the ABS system during low speed stops.

Brake technology, like everything else automotive, is changing constantly. By keeping up on changes like these, you're likely to avoid unneeded comebacks.
Jay M. Buckley Bendix Answerman and Brake Training Administrator Honeywell Friction Materials

Remedy For Pulsing Brakes.
Popular Mechanics; 2/1/1999; WEISSLER, PAUL
Pulsating automobile brakes is a problem that needs attention, beginning with finding the source of the problem. A common cause of brake pulsation is uneven lug nut torque, in which the torque difference should be no more than 20% between each lug nut. Steps for a complete inspection and remedies are described.
The driver in the left lane suddenly realizes he's about to miss his exit, so he cuts in front of you. You mash the brake pedal, and it vibrates like a running chain saw as the antilock braking system (ABS) keeps you out of a skid. Thankfully, you don't rear-end the jerk's car, as he also brakes to slow down. He's on the off ramp and gone, so you don't get the chance to pull alongside and express your feelings with universal sign language. But that's probably a good thing.

While it can be reassuring to feel that ABS-connected brake pedal pulsating beneath your firmly planted foot, it's reason to suspect a problem if you get the same pedal pulsation with a light to medium braking application.

ABS-esque
If the brake feels like the ABS is cycling, but you know that it's not, check the individual axle trigger wheels in front (and on some cars also in back) and the adjacent wheel speed sensors. If you see a damaged wiring connector, bent sensor mounting bracket or stone-damaged trigger wheel, that's a likely cause. But on almost all cars, the cause normally is something in the service brake itself, and the primary problems are ones you usually can check and correct yourself.

Before you check even the most likely problems on a late model, make absolutely sure there isn't a factory problem with a specific fix. Sure, this is low on the "likely" list, but it's a lot easier to look for a bulletin than to pull wheels, etc. If you have a POPULAR MECHANICS CD-ROM disc that includes the service bulletins for your car, you can make absolutely sure. Otherwise, go to the AllData Web site, a leading supplier of CD-ROM information systems for professional mechanics (www.alldata.com). If you work your way through its consumer information section and technical service bulletins (TSBs), you can check the titles of all bulletins for your specific make and model car.

Lug Nut Torque
The No. 1 cause of brake pedal pulsation is uneven lug nut torque. The usual reason is that some mechanic overtightened the wheel lug nuts with an impact wrench, distorting the brake disc.
How do you know if your torque wrench is accurate? Well, if it's by a name-brand tool company and hasn't been thrown around or had a cement block dropped on it, it should be okay. Check the torque at each wheel and compare the reading with manufacturer's specifications (do you have a POPULAR MECHANICS CD-ROM service manual?). If there's a difference of 20% between any two lug nuts, that's too much. Try loosening all the lug nuts, cleaning the stud threads with a wire brush, lightly coating them and the nut chamfer with clean engine oil, and then reinstalling the nuts fingertight.

Next, using a crisscross pattern, tighten all of them to about one-third the specified reading, then to two-thirds and finally to the specified torque.

Basic Diagnosis
If simple retightening doesn't fix the problem, particularly if all wheels were off torque specs, you'll have to pull the wheels for a closer inspection. First, isolate the problem to the front or rear wheels. Try driving the car and then slowing or stopping with the parking brake lever lightly pulled up and your finger on the ratchets release button. If the car has a pedal parking brake, try applying that lightly, but be sure to pick a safe, deserted parking lot with smooth pavement. If the car slows down smoothly to a stop, the brake problem is in the front wheels. If the car decelerates in surges, one or more of the rear brakes is out-of-true somehow.

Brakes Not Releasing
When there's an uneven thickness problem on the discs at relatively low mileage (under 50,000), accompanied by short lining life, it's possible the brakes aren't releasing fully, allowing the shoes to stay in contact with the discs. A defective caliper piston (sticking when you try to push it back), a bad piston seal (not retracting the piston) and a sticking caliper are all possibilities.

When the caliper doesn't slide smoothly back and forth, that's trouble, and short lining life from failure to retract the shoes completely is just one consequence. The brake shoes may also slap unevenly against the disc, another cause of pulsation.

Most calipers slide along guide bolts, through bushings with plastic or metal sleeves. When you remove the caliper, inspect everything. If the guide bolts are corroded, replace them with brand-new ones, lubed with silicone grease (don't try cleaning them). Ditto (including silicone lube) for the sleeves if they're corroded or cracked, and the bushings if they are deteriorated.

Rear Drum Brakes The rear drums are not as frequent a cause of pulsation as rear discs, but if they're warped, or if the linings are not making good contact, they certainly can be responsible. Unless a drum contact surface is badly grooved (grooves here are not as acceptable as on a disc), you'll need a drum gauge to check for taper or out-of-round. Or ask a machine shop to measure it for you, as the typical shop will do it for free or a nominal charge. Linings that are worn unevenly are a tip-off to a drum that should be measured.

Also check for broken springs, or springs that show obvious signs of weakness by allowing shoes to move easily.

Invest in a good torque wrench, keep it in the trunk and use it to tighten the lug nuts instead of the lug wrench when you have a flat. You should enjoy smooth moderate stops for the life of the car.

Tire imbalance, per se, shouldn't cause a pulsating brake pedal. Brake pedal vibrations are almost always caused by an out-of-true condition somewhere in the braking system. But on rare occasions it can happen. Normally, each tire has to be balanced statically first--i.e., so no part of the tire is heavier and always sinks to the bottom. Then they all need to be balanced dynamically, so the rim doesn't wobble at higher speeds. Any imbalances from either of these conditions normally will manifest themselves as steering-wheel vibration, not brake pedal vibration. Even a tire with a high or low spot will normally be felt through the wheel.

But once in a great while you'll find a tire that has inconsistent stiffness. Some parts of the tire's sidewall will be more or less easy to deflect under load than others. This won't show up on the wheel balancer, but will make a brake pedal pulsate at a rate proportional to your road speed. The diagnosis is to swap for a different tire and see if the problem goes away.

Rotor Rehab
Servicing today's rotors involves new techniques, equipment and updating shop skills.
Back in the 1970s, when cars and trucks with disc brakes first began appearing in service bays in numbers, servicing brake rotors was a relatively simple job. Most vehicles in your customers' bays back then were from the Big Three in Detroit, which meant they had rear-wheel drive: discs on the front only and brake rotors that were integral with the hub.

In those days, resurfacing rotors was a regular part of every brake job. They were easily removed from the spindle after removing the dust cap, cotter pin and castle nut. After removing the wheel bearings, the rotor was mounted on the brake lathe arbor. The lathe's adapters used the hub's wheel bearing races to locate the rotor on the arbor, and in most cases, whatever runout that was in the rotor while on the vehicle was duplicated on the lathe. The rotor was machined on the lathe, and runout was usually within specifications when the rotor was reinstalled on the spindle. The rotor also could be turned without worrying much about surface finish, since the asbestos brake pads of the day were tolerant of rough finishes.
Those brake pads also were easy on rotors. That fact, along with the rotor's generous dimensions, allowed a brake rotor to be resurfaced several times. Unless the customer drove the vehicle until the rivets or backing plate left deep scores, it was not uncommon for rotors to last for most of the life of the vehicle.

After the rotors were turned, it was just a matter of repacking the wheel bearings and installing a set of pads, and the front brakes were good for another 30,000 to 40,000 miles.

Complications
During the past three decades, however, changes in vehicle design and materials have directly and indirectly complicated brake rotor service.

The move to front-wheel and four-wheel drive vehicles, as well as the increased use of rear-wheel disc brakes, has made rotor removal more difficult and time-consuming on many vehicles. On some front-wheel drive vehicles, the entire knuckle must be removed from the vehicle and the rotor/hub assembly pressed from the knuckle, with reassembly usually requiring new bearings.
Even the easily removed non-integral rotor "hats" present their own challenge. Because these rotors are machined separately from the hub, care must be taken during rotor setup prior to resurfacing if a traditional bench brake lathe is used, or lateral runout can be excessive when the rotor is reinstalled on the hub.

The quest by the OEMs to reduce vehicle weight, increase fuel mileage and improve ride quality led to the introduction of composite rotors. Composite rotors have a stamped steel center hat attached to a cast iron rotor and are about 20 percent lighter than conventional one-piece cast iron rotors. The downside to these rotors is that they must be properly supported when resurfacing, or they can flex and cause surface finish problems and excessive runout.

Changes in disc brake pad material have affected the brake rotor friction surface. The semi-metallic compound pads that first replaced asbestos were noisy and harder on rotors, with a subsequent decrease in rotor life. The newer semi-metallics, along with the latest non-asbestos organic and ceramic compounds, require a smooth surface finish, meaning that attention must be paid to the condition of the lathe bits and the feed rate of the lathe. Sanding with fine sandpaper after turning is usually required, too.
Regardless of the complexities that have been added to brake rotor service, the goal remains the same: to ensure that lateral rotor runout, rotor thickness variation (parallelism) and rotor surface finish are within the manufacturer's specifications. Today's technician has several ways to achieve this goal, but before we discuss how he does that, the rotor must be thoroughly inspected and measured.

Inspection
Rotors need to be visually inspected for cracks, scoring, heat checks, hard spots and lining deposits. If there are small heat checks and surface cracks, the rotor can be reused providing they are removed by resurfacing. However, large cracks are cause for rotor replacement. Hard spots can cause uneven wear and pedal pulsation and also are a reason for rotor replacement, because resurfacing seldom removes the entire hard spot.
If the rotor passes the appearance test, technicians should measure the rotor thickness and thickness variation using a disc brake micrometer. Unlike a standard micrometer that uses a flat surface on the anvil and movable spindle tips, a disc brake micrometer uses pointed tips. The reason for this is so the instrument can fit into the grooves of scored rotors to accurately measure the thickness.

The rotor thickness should be measured at eight equidistant points around the rotor, at the inside, middle and outside areas of the brake pad contact surface. Most rotors come with the "discard" or "machine-to" dimension cast into them; if this cannot be found or is illegible, the vehicle service manual likely will have the proper specification. A rotor should be replaced if it is worn below the "discard" thickness or if it cannot be resurfaced without exceeding the "machine-to" dimension.

Thickness variation is much harder to measure, because in many cases, anything exceeding 0.0003 to 0.0005 inches is beyond specification. Service dealers should always consult the vehicle service manual for the exact specification. Although this a very small amount, any more can cause pedal pulsation.

Thickness variation usually is caused by excessive lateral runout (wobble). As the rotor turns, the brake pads wear the high spots on each side of the rotor, gradually wearing the rotor until those areas are thinner than the rest. If there is excessive lateral runout at the time of the last brake job or from when the vehicle was new, pedal pulsation caused by thickness variation may not develop for 3,000 to 7,000 miles down the road.

After measuring the brake rotor thickness, the runout should be measured using a dial indicator. If the vehicle is equipped with adjustable tapered roller bearings, all the wheel bearing end-play should be eliminated with the wheel spindle nut before checking the runout. Non-integral rotors should be secured to the hub using the lug nuts installed backwards. The dial indicator can then be mounted on the suspension at a convenient place so that the indicator stylus contacts the rotor face in the middle of the brake pad contact area. Once the dial is set to zero, the total indicator reading can be checked while turning the rotor one full revolution. The reading can be compared with the given specifications.

Runout exceeding 0.003 inch can cause pedal pulsation. Excessive lateral runout can be caused by improper lug nut torquing, a poor previous resurfacing job, rust buildup between the hub and rotor or manufacturing defects.

To cut or not to cut
By now it should be easy to determine if the rotor requires replacement or if resurfacing is necessary to bring the rotor within specifications. But what if all measurements were within specification, the friction surface is smooth and the vehicle had no brake problems other than worn linings? Should the rotor still be resurfaced? Unlike the old days, today the prevailing wisdom is "no" for several reasons.

Resurfacing the rotor unnecessarily removes material and makes the rotor thinner, lessening its ability to absorb and dissipate heat and shortening the rotor's useable lifespan. Improper machining also can create problems. For instance, if a non-integral rotor is not setup properly on a bench lathe, an excessive lateral runout condition that did not previously exist can be created when it is reinstalled on the hub. Improper resurfacing also can create a friction surface that is too rough, causing brake noise, premature pad wear and a hard pedal condition.

Many of the OEMs have issued bulletins in the last few years providing guidelines for brake rotor servicing. In these bulletins, they've indicated that brake rotors should not be resurfaced during routine brake pad replacement. Most also recommend that new rotors not be resurfaced.
Finally, if the rotor's surface condition is acceptable and all dimensions except runout were within specification, the technician should try changing the position of the rotor on the hub and rechecking the runout. A change in position may bring runout within allowances.

If inspection and measurement determined that resurfacing is necessary, there are two alternatives: the traditional bench lathe or an on-car brake lathe. As stated previously, a traditional bench lathe is fine for resurfacing rotors with integral hubs and tapered roller bearings. However, a bench lathe requires more effort for turning non-integral rotors or excessive lateral runout can be created. After measuring runout with the rotor on the hub, the runout also must be measured with the dial indicator when the rotor is mounted on the lathe. If the runout isn't in the same place and/or the same amount, the rotor should be shimmed until the runout is duplicated.
Another way to use a bench lathe and maintain lateral runout within specifications is to use tapered shims. After the rotor is resurfaced, it is remounted on the hub, and runout is checked with a dial indicator. If runout is excessive, the high spot is marked and the rotor is removed. The tapered shim is then installed on the hub with the thickest part opposite the high spot. When the rotor is reinstalled, runout should be within specification.
When using a bench lathe to resurface composite rotors, special adapters must be used to support the steel center section, or the rotor can flex and cause runout and surface finish problems. Also, if an aftermarket cast replacement rotor is used to replace a composite rotor, the rotors on both sides of the vehicle should be replaced, regardless of the other rotor's condition.

For a bench lathe with a fixed 150-rpm spindle speed, a rough cut of 0.005 inches can be made at 0.006 to 0.010 inches cross feed per revolution. A finish cut of 0.002 inches should be made at no more than 0.002 inches cross feed per revolution.

On-car brake lathes fall into two categories--caliper-mounted lathes and hub-mounted lathes. Both types are ideal for resurfacing rotors on vehicles where the rotor is difficult to remove.

Caliper-mounted lathes were the first type of on-car lathes widely available. They are two-piece units with one part rotating the rotor and the other part being the cutting mechanism, which is attached to the steering knuckle. They usually require many adapters due to the variety of steering knuckle designs, and they can be time-consuming to set up.

Hub-mounted lathes combine the rotor drive device and cutting mechanism in a single unit. These lathes machine the rotor so that it is perpendicular to the wheel's axis of rotation, thereby minimizing runout. Most require only five or six adapters to allow hub attachment on most cars and light trucks. The latest design hub-mounted lathes can resurface a rotor in approximately 10 minutes, from the time the machine is attached, to the finished rotor. Some offer automatic runout compensation, which further simplifies the setup process.

Regardless of the type of lathe that is used, the finished surface of the rotor must be very smooth: between 20 and 70 micro-inches. After the rotor has been machined, the friction surface should be sanded with 120- to 150-grit sandpaper, mounted on a sanding block, while the rotor turns on the lathe. A non-directional finish should be applied using the sandpaper, until the machined finish that was created by the cutting bit begins to disappear, about one minute per side.

After resurfacing
Now that the brake rotor is within specification and ready to be put back into service, attention to detail during the remainder of the brake job will help ensure that the rotor stays true and the brakes perform properly.

When rotor resurfacing is completed, the rotor should be washed with soap and water, and wiped off with a clean shop towel. Brake cleaning solvent may not remove all the fine particles left over from the machining process, and these can become imbedded in the new pads and cause brake noise. The inside of a non-integral rotor and the hub flange should always be clean and free of corrosion, dirt or burrs that can cause runout when reassembled.

Although disc brake noise is usually thought to be caused by the disc brake pad against the rotor's friction surface, in reality more noise is caused by the brake pad not being insulated properly from the caliper or caliper piston. All required disc brake pad shims, clips and anti-rattle springs must be installed properly. In addition, old disc brake pad shims should not be reused and should be replaced along with the brake pads.

All other caliper hardware like slide pins and bushings should be checked to make sure they are in good condition and questionable parts should be replaced. The caliper slides and shoe pads must be cleaned and lubricated with moly-based high-temperature brake grease.

The lug nuts should be torqued in a star pattern using a torque wrench or torque-limiting socket. Lug nuts should be tightened in two steps--first to half of the torque specification and then to the full torque specification. The new pads can be broken in by making about 10 moderate stops from 30 mph, waiting about a minute between brake applications, and avoiding severe braking for several hundred miles.

Mike Grady is a technical writer and former senior editor at the Chilton Book Co. He is an ASE Master Engine Machinist and also holds ASE technician certifications. Grady worked as an automotive machinist for 10 years and has more than 24 years experience in the automotive industry.

Hope this helps everyone.