Warped rotors. There is no such thing as checking them with a straightedge. I'm not the best at describing the different types of warpage but I'll give it my best shot. Imagine if you had the rotor mounted on a brake lathe. The cones that hold it grasp it by the mounting plate right around the stud holes. That is the surface that it must be indexed on because that is what will sit flat against the hub on the truck. Now, two cutting bits, one on each side, remove the high spots and make each braking surface perfectly parallel to that mounting surface. Along with that, the thickness between those two braking surfaces is exactly equal all the way around.
The first type of warpage is thickness variation. That must be measured in six to eight places around the rotor with a micrometer. As the rotor turns, when that thicker spot goes between the brake pads, it pushes the pistons back into the calipers. That pushes brake fluid back up to the master cylinder and pushes the brake pedal up against your foot pressure. When the thinner part of the rotor goes between the pads, the pistons can come out, brake fluid takes up the space behind the pistons, and the brake pedal goes back down a little. .003" thickness variation is plenty to feel in the brake pedal. That is the thickness of a sheet of paper.
The second type of warpage is a whole lot easier to describe with a group of students standing around the lathe so I can point to things. Because you have a 4wd truck, you have rotors that slide on over the studs. Imagine it is mounted on the lathe but there is a piece of rust or scale stuck between one of the mounting cones and the mounting surface of the rotor. The entire rotor will wobble as it rotates. Now you machine both sides. When you're done both sides will be perfectly parallel to each other, there will be absolutely no thickness variation, but the entire braking surfaces are not parallel to the mounting surface. When you mount the rotor on the nice true hub on the truck, the braking surface will have "lateral runout". If it's bad enough to see, you will see the brake caliper moving left and right as the rotor goes around. There is no thickness variation so you won't feel anything in the brake pedal. All brake rotors develop a little lateral runout but if the caliper is free to slide left and right, it is unlikely you will feel it. I looked up your caliper mounting system, (here's a link to what you should have):
Your calipers will slide back and forth on the silver tubes. I had to go all the way back to 1993 models to find the wedge insert. Here's what they look like:
Even that style of caliper retainer is relatively forgiving because there is rubber sandwiched between the two metal parts. That rubber lets the caliper move back and forth a little with the lateral runout in the rotor.
To add to the confusion, there are two different times warpage can appear in a rotor. Most people assume it can only happen on the truck from them becoming hot, but it can also be machined in accidentally on the lathe. I already mentioned the chip of rust or scale getting stuck behind the mounting plate. That will most definitely put lateral runout in the rotor. A much less known cause is over-tightening the nut on the end of the shaft. The mounting cones that hold the rotor are never perfectly true but that doesn't usually cause a problem if the nut is just snugged up. Some mechanics think that nut must be tightened as much as possible, but that will bend the shaft of the lathe by forcing the cones tighter together. That is very easy to see when the lathe is spinning. The end of the shaft will appear to move up and down as it rotates. That WILL result in lateral runout.
If the caliper is free to slide on the mounting sleeves, you will likely not feel that lateral runout, at least at first. Eventually any brake grease will wear off, and dirt and water will get in there and reduce the caliper's ability to slide freely. If the caliper refuses to slide, it will force the rotor to move sideways a little to center itself between the brake pads. Tugging on the rotor means it's also tugging on the hub the rotor is mounted to. That tries to turn the wheel and is why you will feel that tugging in the steering wheel.
There is a third thing you must look for on your rotors, if this applies. Rotors that slide onto the hub, such as yours, often have an access hole in the hub itself. There are the five, (or six) holes for the wheel studs, but there is often an additional hole that allows you to reach the bearing assembly mounting bolts with a socket and extension. Water will spray up in there and form a small circle of rust on the back side of the rotor. This is extremely common on all brands of vehicles. That rust spot, (sometimes three of them), must be scraped off before the rotor is machined. The mounting cone will rest on that rust and cause the rotor to sit crooked on the lathe. That will result in lateral runout being machined into it. Even if the cone doesn't rest on that rust, the rotor can be reinstalled five, (or six) different ways. If that rust spot doesn't line up with the hole that let it form, it will be wedged between the hub and rotor's mounting plate. You might not notice that right away on a heavy truck but you WILL feel it on a light weight car. This was a real common problem on GM cars. It wasn't as noticeable on Chrysler cars because they had three holes and three spots of rust buildup so it was more likely the rotor would still mount parallel to the hub.
The only way to measure lateral runout is with a dial indicator as shown in this link:
The stem near the lower left side is placed perpendicular to the braking surface and partially pushed in, then you watch how much the pointer moves as the rotor spins. This model can measure a total of one inch. That is ten revolutions of the pointer. There's 100 graduations around the outside. Each one represents .001". It is common to find .003" to .005" runout in any rotor. By the time you get to .010" you will probably start to feel it in a small car but not necessarily yet in a truck.
When doing this on your truck the lug nuts must be installed to hold the rotor in place the same as it will be when the wheel is installed. We don't normally have the time to do this but if a mechanic is really conscientious, he will measure lateral runout this way before he removes the rotor for machining, then he will measure it again when it is mounted on the lathe. You know the hub on the truck is true so if the rotor is mounted properly on the lathe, you should measure the same amount of runout. That extra step is usually skipped because it's so seldom we run into a problem. It's the one in a hundred that give us trouble where we need to resort to this type of test.
After performing a brake job on a mid '90s Dakota many years ago, I accidentally machined in severe lateral runout. That one used the same type of slide-on rotor that you have. The only hint of a problem on the test drive was a clicking sound once per tire revolution when the brakes were applied. After lots of frustration, it turned out to be one inner pad sliding sideways along with the caliper and catching on a notch in the mounting knuckle. That notch was the result of rust and wear over many miles. While running it in gear and jacked up, you could easily see the caliper moving sideways about 1/16" as it tried to stay centered over the rotor. There was no unusual feeling in the brake pedal or steering wheel. Had I ignored the noise, it would have eventually gone away when the lining wore down and the metal backing plate became centered in that fairly wide notch.
As far as your question about checking the calipers' ability to slide, that shouldn't even be an issue with your truck. "Newer" to me means anything newer than a late 1980s model which is what I drive. In the '70s Ford used steel inserts with a steel spring-metal wedge to hold the calipers. You actually needed a large hammer to slide the caliper sideways. The idea was that as the outer pad wore down, the caliper would eventually slide from heavy pedal pressure. Since it would never slide back, the outer pads wore very quickly if you always used the brakes hard. If you were light on the brakes, only the inner pads would apply and wear faster. That was a very poor design. When I worked at a Sears Auto Center in the '80s we saw a lot of those trucks come in with grinding brakes at less than 15,000 miles.
The rubber wedges shown in the second link above is an improvement but why did the engineers think the caliper would slide to take up the pad wear when the rubber in that insert could just bend, and then straighten out when the pedal was released? Uneven pad wear was still a big problem. The best design is what you have on your truck. With the caliper still mounted in place, you should be able to easily pry the pistons into the caliper with a large flat blade screwdriver. If you can not, the piston could be sticking. That's a whole different story for another day. Once both pistons are retracted, you might be able to slide the caliper by hand. Don't panic if you have to bang it with a small hammer. It's sitting on rubber inserts that ride on those metal sleeves so it will stick a little. It will slide over time to accommodate the outer pad wear, and it will slide a lot easier than either of the previous designs.
Tuesday, February 22nd, 2011 AT 1:30 AM