Whether or not to do this yourself depends on your level of experience, skill, and confidence. You would expect common sense to be the main requirement, but I can offer you some always-overlooked details the specialists do that aren't common knowledge.
First of all, understand the manufacturer has put a real lot of time and research into providing optimum front-to-rear brake balance based on the weight distribution of your exact car model and the optional equipment placement. That balance is affected by the surface area of the brake linings and the "coefficient of friction" of the material used. Even the diameter and thickness of the rotors affects how quickly they heat up and that temperature changes the coefficient of friction. The goal is that change affects the front and rear brakes equally. All you need to know is every brake pad manufacturer designs their linings to meet the same standards for coefficient of friction. If the linings use a material that has more friction, or grabbing power, the physical size of the linings will be smaller. It takes just as much work to manufacturer cheap brake pads as expensive ones, so there's little point in trying to save a few pennies by using inferior material. If there is an inferior product on the market, it's doubtful you'll find it at an auto parts store. At the very nice Chrysler dealership I used to work at, we got aftermarket pads made in our state that cost less than what you'd pay at an auto parts store, and we had very good luck with them.
For the rotors, it's getting hard to find any that are not made in China. There is nothing wrong with their quality, but warping is a real common problem. That's because when we make parts from cast iron, we set them aside for up to 90 days to "age" before they get their final machining. The Chinese cast 'em, pack 'em, and ship 'em, then they age on your car. Warping a couple of months later is common, and a light machining will solve that permanently. If you buy these online, you might save a couple of dollars up front, but you'll spend more later having them machined. When you buy them from an auto parts store, they will often machine them for you for free later. Some people get angry and demand new ones, but they don't understand they're going to have the same problem or why.
Also related to rotors. It has always been standard procedure to machine the old ones to true them up as part of a normal brake service. The cost of cutting bits for the brake lathe and the cost of maintaining that equipment meant we had to include dollars in the repair cost to cover those expenses. The cost of replacement rotors has come down so much that now it's less expensive for you and for the shop to just replace them. Also, to save weight, most original rotors are made so thin that it just takes a little normal wear to make them below the published legal minimum thickness. No mechanic will risk a lawsuit by returning under-size rotors onto a customer's car. Installing new rotors removes the chance of measuring an old one incorrectly by mistake, or finding out that it takes too much machining to true it up and they have to tell you more parts are needed than were first quoted in the repair estimate.
With pads and rotors, I think I would steer clear of online parts. You want parts from someone you can ask for advice, or in the worst case, throw something at if you're unhappy! Complaining to your computer won't solve anything.
If you aren't having a braking problem now, particularly a pedal pulsation, it is not absolutely necessary to have the old rotors machined, but you need to understand why we do that so you can make an informed decision. Machining is much more important for front rotors because they do about 80 percent of the stopping and generate the most heat. The obvious reason is to insure the friction surfaces are perfectly parallel to the new pads and are perfectly flat with no grooves. When there's a groove, that part of the rotor doesn't contact the lining until the rest of that lining wears down. In effect you have a smaller lining that doesn't do its share of stopping. In severe cases, you don't even notice that you have to push harder on the brake pedal to slow down, and that parts of the linings that ARE making contact heat up much too fast. That can lead to brake fade which can occur suddenly and without warning, so it's a safety issue. The fix for that is to stop driving until the linings cool down for an hour, then they're typically fine after that.
To avoid that brake fade and reduced pad contact, we machine the rotors, but that leaves them with very fine grooves like on a record. During the test-drive, the mechanic will perform a few pretty hard stops to wear the pads down so they develop grooves to match those on the rotors. A lot of shops hang notices from your mirror informing you to go easy on the brakes for the first hundred miles until the pads have "seated" and match the rotors. Most of this issue is avoided with new rotors because they are prepared with the specified "surface finish" to reduce brake squeal and shorten the brake-in period.
The other reason for machining old rotors is the rust buildup on the inner and outer edges of the friction surfaces. No two linings are glued to their metal backing plates in exactly the same orientation, so the new one is likely to ride on a ridge of rust until that rust wears down. That occurs very quickly, but customers will find that objectionable when they drive away from the shop.
If you do have your old rotors machined, the mechanic will measure them first to see if they will likely be thick enough to put back in service, and he will measure them again after the final machining. There is a published "machine to" specification that he can't go below, and there is another "discard" specification the rotor can be allowed to wear down to after it has been put back on the car. There used to commonly be.030" difference between those specs. If a rotor is warped, which is typical, it can appear to have enough material left to allow machining, but a lot of metal may have to be cut from one side to true it up before the cutting bit makes full contact all the way around it. That results in the rotor ending up a lot thinner than what the original observation and measurement said it should be. That's another reason for just replacing them instead of being surprised during the machining process.
When old rotors are machined, look on the back side where the center contacts the hub. You'll see one or three round raised spots of rust from water that splashed through the access holes in the hub surface. Those have to be cleaned off. They will prevent the rotor from sitting squarely on the brake lathe so a warp will be machined into it. They will also prevent the rotor from sitting squarely on the hub and you'll have a brake pedal pulsation, AND a wobble from the wheel which also won't be sitting squarely. The ring of rust must also be scraped off the inside of the rotor next to the cooling fins where it meets the outer circumference of the hub. That rust and scale can break off during installation and get trapped between the hub and rotor. That also leads to the rotor not sitting squarely on the hub.
A light film of high-temperature brake grease needs to be spread inside the center hole of the rotor to prevent causing a crunching noise when cornering. Brake grease is made for this purpose. It will not travel like axle grease does. This is almost always overlooked by do-it-yourselfers. That grease also must be used between all the pad to caliper contact points and caliper to mount contact points. First, run a flat file over the piston and the fingers on the caliper that contact the pads' backing plates. That is to remove any rust, dirt, or previous material that was applied to stop squeals. The pads must make full contact and have that lubricant to avoid squeals. Pads are going to vibrate, and that can't be stopped, but we can stop that vibration from transmitting to the caliper where it will be amplified and heard as a squeal.
The rotors have to be cleaned too with Brake Parts Cleaner to remove dust from machining and from old lining material. Any of that left on there will become impacted in the new linings and will set up a squeal. The same is true of grease contamination. Most specialists only hold machined rotors by their edges, never by the friction surfaces, and they typically wash their hands first to remove fingerprint grease. Any grease that does get on the linings or rotors can be washed off right away with Brake Parts Cleaner, but if those parts go through a single warm-up cycle, the grease will soak in and never come out. That pertains to the linings and the rotors. Cast iron is porous and will soak up the grease which will lead to a squeal. The only proper fix for that is to replace the linings and rotors.
A little-known trick to avoid those irritating squeals is to prepare the pads by removing the sharp edges on the leading edge of the linings. That removes the "fingernails-on-the-blackboard" squeal. I used to hold my breath and grind the edges on a bench grinder. Later I just used that flat file to break the edges to about a 45 degree angle. I found that you just have to remove enough material to prevent squealing during the break-in period, then the problem wouldn't occur later when the linings wore past what you filed off. Five or six passes with the file are sufficient. Some people will argue doing that removes the linings' "squeegee" effect that removes water to prevent a different kind of brake fade after driving through deep water, something disc brake pads are famous for over drum brakes, but then you can counter with the fact that a lot of new pads come with their edges ground down a REAL lot more than that. It used to be common on GM front-wheel-drive cars to see over an inch of material ground away by the manufacturer of those linings.
You're going to have to push the piston into the caliper to make room for the new, thicker pads. That's easiest to do with a flat blade screwdriver as a pry bar before you unbolt the caliper. Some people do that with a C-clamp, but if you NEED a clamp to force the piston in, forget it and replace the caliper. If it goes in that hard it's because rust and dirt have built up on the piston and you're trying to push that under the rubber square-cut seal. That is going to make that caliper not apply properly under light braking and not release after normal-to-hard braking. The exception to this is if your parking brake is built into the caliper. For those you need a special tool to turn the piston and screw it into the caliper. Those are readjusted at the end by exercising the parking brake to work the pistons out.
Speaking of working the pistons out, for all front calipers and the rear ones that don't have the parking brake built in, you run the pistons out to adjust them by pumping the brake pedal. Do not push the pedal more than halfway to the floor when doing that or when bleeding a brake system. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. Running the pedal past that point runs the lip seals over that crud and can rip them resulting in a slowly-sinking brake pedal. This is not a problem yet on a car less than about one year old or with a recently-replaced master cylinder.
Any flat mounting surface on the knuckle that a caliper or pad rests on must be cleaned up with the flat file too, then be coated with brake grease. The caliper slides very little during brake application and release, and it slides a total of less than a half inch over the life of the linings, but it has to be able to do that freely. Grooves will wear into those surfaces too that inhibit free brake application later in the vehicle's life. Brake grease reduces the chance of that happening. If your calipers are mounted on a pair of bolts, those bolts must be replaced if they're bent of if the chrome plating has lifted. Lifting of the chrome plating results in rust spots that prevent proper operation of the caliper, and sanding them off will not be a permanent solution. Coat the bolts too with brake grease.
When you have the calipers off, never allow them to hang by the rubber flex hoses or fall down and yank on them. That can tear the hose internally inside the metal crimp. If the outer casing of a hose is cracked to the point you can see the white reinforcing string inside, it should be replaced. There's a white stripe on the hoses to show if it's twisted. If you twist the caliper one complete revolution after not removing the hose, that twist will be real obvious and you probably won't be able to mount the caliper. On some older cars, Fords most commonly, they had front hoses that were screwed into the calipers and could be turned in the wrong amount that caused that twist, but you could install them that way. That creates the possibility of a hose bending in such a way that it could rub on the tire.
For my final comment, if you ever need to do anything that involves brake fluid or the parts that contact it, you must never get the slightest hint of a petroleum product in there. That includes engine oil, transmission fluid, power steering fluid, and axle grease. Professionals even wash their hands first with soap and water when working with those parts to prevent contaminating the system with fingerprint grease. A little film of oil on your fingertips when resetting the rubber bladder seal in the reservoir cap is enough to contaminate the entire system and cause problems. The only permanent fix for that is to remove every part that contains a rubber part that contacts brake fluid, flush and dry the steel lines, then install all new rubber-containing parts. That gets to be real expensive, and if the car has anti-lock brakes, with its hydraulic controller, it can make the car not worth repair.
Monday, October 27th, 2014 AT 11:31 PM