Odd front brake wear pattern

While doing my left front wheel bearing I got a closer look at the brake rotor than ever before. Since buying the car 5½ years ago I've never done more than look at the visible disc surfaces and look through the calipers at the pad thickness, and the pads have stayed plenty thick even up to 210,000 miles.

But taking the left front disc off, I noticed a strange wear pattern on the inboard side, where it was never easy to see. It's as if the pad has only been touching in a narrow path down the middle, with the inner and outer edges rough, rusty, and untouched. The inboard pad is still plenty thick, but worn into a stepped shape where the two concentric edges are missing more material than the middle. That made me want to take off the right disc too, which showed something of the same pattern, though not quite as bad:



In trying to explain this to myself I run into a chicken-and-egg problem. I can definitely see how, if the discs had these rough rusty edges, they would chew up the pads at the edges and make the problem self-perpetuating. Or I can see how, if the pads had this step-shaped wear at the edges, they'd leave the disc edges to get rusty and rough, eventually enough to keep chewing the pads in the same pattern and make it self-perpetuating. What I don't see is how either one of those ifs got started without the other one.

So, off went both discs to the shop for turning at the same time the bearing went for pressing. When the shop was done with them, they still have 36% of the wearable thickness remaining, not bad for 210,000 and probably good for a long time yet - if they don't start wearing this odd way again.

In addition to new pads (04465-17140), I figured possibly something was binding with the springy "support plates" that hold the pad edges (sold as a "fit kit" 04947-47010, about $11), and also replaced all the shims (04945-20210), perhaps unnecessarily.

I also noticed the rubber boots at the ends of the front suspension stabilizer links had cracked and started leaking grease, so now there are shiny flat rotors and shiny new stabilizer links along with the pads and springy bits. I'm sure I'm probably due for shocks too, but it can't all be in one week....



-Chap

 

Well, yes and no. Yes, let's say you've got nice rotors in good condition, smooth all the way across, and the car has been sitting in damp weather all night, and in the morning you drive away and notice a rough sound from the brakes, that will be light flash rust on the rotor surfaces, and you can wipe it right off by shifting to neutral and braking to a stop. No, you don't have to be cruising down the highway, and you don't have to "slam" on the brakes. All you have to do is shift to neutral and use them normally, like at the next stop sign on your neighborhood street. It doesn't have to be anything a passenger in your car would notice you doing differently, unless they happened to see you shift to neutral.

But also no, if your rotors have already rusted to where they look like mine in the photo, that isn't flash rust that the pads can just wipe off. When it gets to that stage, it grinds away the pad surface instead of vice versa. So the pads aren't flat across anymore, and if you were to have the rotors resurfaced on a lathe but put them back with the same pads, the pads would only touch in that middle part, and the rings of rust would come right back, because the pads aren't touching there.

So once it's like that, it can only be fixed by getting back to flat rotors and flat pads.

Sticking caliper pistons would tend to cause brake drag and heat. Sucky mileage, shortened wheel bearing life, cooked rubber, but probably not rusty rotor surfaces. If anything, I'd expect them to be rather well polished.

There is something to the regen braking, just because it means the real brakes don't get as much use. That's why the occasional brake (not slam) in neutral is helpful; neutral disengages the powertrain, so regen can't be used, and the car is forced to do your braking using the brakes.

I honestly don't know what gets that condition started. My Gen 3 is pushing toward 150,000 miles now and is not getting like that; I check every time I rotate tires. I did not check my Gen 1 as often, and that was part of the problem. I have not had to do anything on the Gen 3 when I inspect to keep that pattern from starting; I just check whether it's starting, and it isn't.

If you find the rotors starting to get rusty or pitted or shiny, but nothing deep, you can clean them right up with some 100 or 150 grit wetordry sandpaper, wet, using a pad as a sanding block, then turn the paper over, laying flat on the rotor, and rub the pad around on it to deglaze the pad surface. More serious measures like cutting on a lathe aren't needed unless the surface defects are worse than that. Those big gnarly rust rings, of course, do call for a lathe, or new rotors.

 

On my car (whether my former Gen 1 or my current Gen 3), if I use no special tactics, the sound will dissipate by maybe my second day's normal commuting after the dampness event (provided the dampness doesn't return again the next night).

I'm usually not patient enough to put up with the racket for two days, so I use the shift-to-neutral when approaching a stop sign, and it's all quiet by the second stop sign.

 

Either you don't RC, or they weren't describing it clearly, or they were BSing you. Those are single-piston floating calipers (so, all of two pistons "for all the front calipers", all two of them).

The machinist I took my rotors to was very conservative, and stopped removing material when the surfaces were smooth-ish again. but you could still plainly see where the rust tracks had been. I suspect that means it's easy for it to start there again. The rotors had plenty of material left, and I was a little disappointed the machinist hadn't just kept going until the surface was all uniform again.

There are two kinds of special grease for the Prius calipers: the red stuff, for the slide pins and piston and seal (you won't be in there by the piston seal unless you actually disassemble and rebuild the caliper), and the gray stuff, for between the shims, and the shims and the pad. If you're working from the repair manual, you'll see the two different arrow styles pointing to where each type of grease goes.

 

What concerns me when things reach that stage is that the rust there is too well established for the pads to rub it away during braking. Instead, it eats the pads, from the edges toward the middle:




Because it's possible to have Prius brake pads last a crazy long time, it can feel a bit embarrassing any time you end up having to replace them "early" because they were eaten by something correctable.

Note in the upper photo that you can see the pad is being eaten from both the top and bottom edges.

The bottom-edge contour isn't visible in the lower photo because its radius doesn't reach out to the side edges yet.