By now, the majority of technicians know that brake rotor thickness variation is the condition that generates your typical brake pulsation concerns. Hopefully, you also understand that “lateral runout” of the rotor is the underlying culprit. There are many great articles written on the topic of lateral runout and methods of preventing and eliminating it, including, but by no means limited to, cleaning the rotor mounting surfaces, properly torqueing the wheel assembly, and use of an on-the-car brake lathe. In this article however, I would like to examine two atypical causes of thickness variation that can have a “problem vehicle” back at your shop again and again if not addressed (a problem vehicle will typically be back in about 3,000 miles).
First of all, even if lateral runout were corrected to absolute zero, it’s only temporary! As a matter of fact, even after refinishing the rotor with an on-the-car brake lathe, you will probably have some runout right after re-installing the wheel. Unless of course, you torque all the lug nuts to specification by turning them equally and at exactly at the same time, like the factory installs them.
The one cause that is not typically addressed, in no small part due to lack of a method, is the brake caliper not releasing the pads sufficiently, which can really only be caused by two things:
• The caliper slide mechanism binding causing the outside pad to not properly release; and
• The piston sticking in its bore causing both pads to not properly release.
Sure, we all clean and lube the caliper slide mechanism(s), right? But what if for example, a caliper bolt has been cross-threaded or bent, causing the bolt to be crooked? Or, on a sliding caliper, the knuckle is bent so that the caliper does not slide freely enough? How do we check that?
Check The Mechanism
The easiest and most effective way to check this is to first clean and service the slide mechanism, then with piston fully retracted and pads removed, bolt the caliper back on (and, yes, torque the bolts). Then manually slide the caliper back and forth through its full range of travel while keeping it parallel with the rotor. It should slide smoothly and freely from one end to the other.
Check Caliper Piston
Now, before the piston sticking issue can be addressed, we need to make sure we understand the mechanism that causes the piston to retract in the first place.
The caliper piston seal is of a “square cut” design. Note that on the earliest disc brake systems the seal resided in a square groove and the piston did not retract at all. Newer cars, however, have one minor, but very important modification; the outside edge of the groove (the edge of the groove nearest the inner brake pad) is cut at a slight angle (the angle varies between 15 and 30 degrees depending on manufacturer design). (See Figure 1.) This angle allows the seal to flex slightly on brake apply. Then when the brake is released the seal then returns to its normal shape pulling the piston back to its original position. (See Figure 2.) It’s important to note that if the pads have worn enough, the seal will actually make contact with the angled edge and stop. The piston will then continue to move, slipping on the seal allowing the piston to adjust for wear.
Now the question becomes, how do we verify that:
1. The piston is not sticking in its bore.
2. The piston is retracting the desired amount.
For example, we could check for piston sticking by using a spring gauge to measure the force required to push the piston back in. Question is, how much force should it take? There is no service specification published that I’m aware of. But I’ll get back to this in a minute.
Measuring the retraction is actually fairly easy using the following procedure:
Caution: Leave the other caliper(s) installed or place a suitable piston stop in it to prevent the piston being pumped out of the caliper!
With pads removed, brake hose still connected, and piston fully retracted:
1. Mount a dial indicator to the caliper housing and load it 0.020 inches or so (Figure 3).
2. Have a helper push the brake pedal down slowly until the indicator moves 0.020 inches or so, then hold it until the reading on the dial indicator stabilizes.
3. Zero the indicator (Figure 4).
4. Release the brake pedal.
5. Now read the relative movement indicated (Figure 5).
How much did the piston retract? It better be at least 0.002 inches more than the manufacturer published lateral runout specification. The 0.002 inches is based on personal observation after measuring many vehicles, so your mileage may vary. But you got the idea. So, if the piston retracts 0.006 inches, your spec is probably going to be 0.004 inches giving you 0.002 inches clearance when the brake is released.
This checks the piston for sticking at the same time, as it can’t retract properly if it is sticking in its bore. Piston retraction can also be measured at various intervals throughout the pistons travel (I have seen several of them stick in only one area). This works on multiple piston calipers as well due to the fact that it is not the hydraulic system, but the piston seal that is doing the retracting. So as long as you can depress the brake pedal and get the piston to move the 0.020 inches or so you need, if working properly, it should still retract regardless of the action of the other pistons. You may have to block the other piston(s) to get the piston you’re checking to move.
I do not check this on every disc brake job that comes through the door. I’ve found it invaluable, however, for problem vehicles or those with high mileage and/or calipers too expensive to replace simply on a hunch.