Again, this goes back to the '90s, but it may still apply. After doing brake work for many years, we had never run into this as a cause for a brake pull, but now it has become an issue on other truck brands as well.
The service bulletin had us check that matching calipers were in use. There was 80 mm pistons and 88 mm pistons, and standard and low-drag calipers, so there were four possibilities for each wheel. You could tell if the pistons were the same diameter by measuring the width of the calipers' mounting pads.
Next, if you have the suspension system with four control arms that go from the firewall to the front axle, be sure they all have the same type of rubber bushings. That's not a concern unless one was replaced. There's standard bushings and heavy-duty bushings. That has nothing to do with the size or weight of the truck. Look at the lip on the metal sleeves around each bushing. One style has two small indentations or notches that are real easy to feel and see. If you find that on one bushing, you must find them on all eight; (all four control arms).
That same suspension system uses a "track bar" that mounts on the frame on the driver's side and the front axle on the passenger side. That used to cause a lot of steering wander, and you were to inspect it as a possible cause for a brake pull. The newer trucks have an improved design that doesn't cause much trouble.
If you find mismatched control arms or calipers, you'll have a brake pull all the time. Problems caused by the rotors don't show up until they reach a certain temperature, typically after a half dozen fairly hard stops in a short period of time. When the rotors are different thicknesses, they heat up at different rates, and that changes the coefficient of friction of the brake linings. You'll have a number of stops with absolutely no hint of a brake pull, then the next stop you'll end up on the sidewalk! The steering wheel will get tugged right out of your hand.
I've owned cars before that had almost 0.125" difference in the thickness of the front rotors, and never had a braking problem. That's why it was so hard to believe such a tiny difference now can make such a big problem. I was involved with two lemon-law buybacks. Both involved trucks that had recently had a normal and proper brake job done. On the first one, after spending two days checking everything, it came down to 0.020" difference in the rotor thicknesses. Swapping them side-to-side made the brake pull go the other way. On the second truck there was just 0.007" difference. That's like two sheets of paper on a rotor that's over an inch thick. In both cases I machined the thinner rotor to take a light cut, then machined the second one to exactly the same thickness, AND, ... Exactly the same speed of cut. Cutting them on the lathe leaves grooves like on a record, and more or fewer grooves means more or less points of contact, and more or less friction and heat buildup.
One thing that does not correlate with this service bulletin is the pedal pulsation you mentioned. That relates to a caliper that isn't fully releasing. The first place I'd look is the metal brackets in the middle of the rubber flex hoses. They anchor the hose to the axle. If one of those brackets got hit, or if rust builds up inside the crimp, it will constrict the hose. You'll get fluid pressure through there to apply the brake, but that fluid won't be able to release immediately on its own. You'll feel a wobble in the steering wheel.
Saturday, April 26th, 2014 AT 12:21 PM
