Continuous frozen rear caliper and even brake wear

this has been well documented as lack of slide pin lubrication in snow/salt area's, are you sure they are lubricated properly?

 

gotcha, never heard of another problem. what caliper issue do you perceive, frozen piston?

 

What lube was used on the pins?

Also, whoever's opening up the rear brakes, are they aware of the piston orientation requirement? Piston should look (and remain thus, after install) like this:



Not like this:

 

The lube sounds like good stuff, spec'd for pin lube.

Regarding the piston pattern orientation: you want to reassemble with the pattern in the "X" configuration (per first picture above), and then you want to ensure it stays thus. This would entail pumping the brakes multiple times, taking the car for a short test drive, and only afterwards using the parking brake, if needed.

My sense is that if the caliper piston is not well seated and the parking brake is yoinked, it's going to try to rotate the piston, might succeed, and if that happens, then you've got the piston's raised spoke pressing only the pin on the back of the inner brake pad, uneven pressure on the pads, uneven wear, scoring rotors, and so on.

A good test after rear brake work and all of the above steps, is to raise the rear once more, ensure the parking brake is released, and spin the wheels, checking for drag. A disk brake by design will have slight drag, but with moderate effort you should easily be able to spin the wheel one or two revolutions.

 

The piston of a good caliper should extend while you introduce pressure through the brake-line port (there should be something in the caliper about the width of a rotor+pads to stop the piston popping right out), and when you release the pressure, the piston should slip back in by about 0.3 mm or so. You can use a dial indicator to measure it, but 0.3 mm is about visible to the naked eye. 0.3 mm is what I measured on my old Gen 1 front calipers, I don't know if it might be slightly different for the rears.

That's the only thing that "releases" the brakes when you release the pedal, and it's all done by the square-cut rubber seal around the piston (which you can't see without taking the piston out). Gen 3 rears also have the little V-shaped springs to help spread the pads back out, but those only add a bit of help.

That retraction can stop happening as the rubber seal ages. Once that happens, the brakes drag, and get crazy hot, and cook the rubber faster.

If you buy inexpensive rebuilt calipers, you can even end up with ones that you can test right out of the box and measure zero piston return. It may be about the exact materials they use. One I bought that had that problem had a much shinier, polished piston than the more matte-finished one Toyota uses, and maybe that gives the seal less hold on it; they might also use a different rubber in the seal. For a critical part of caliper behavior, it all depends on pretty subtle properties of materials.

Also, I'm guessing Toyota tests theirs before they go in the box, and I'd bet the vendor I bought that rebuild from does, ahem, less of that. So my new policy is, maybe I'm not forever against third-party rebuilt calipers, but any that I buy is going to get tested on the bench before I sink any time into putting it on the car.

If the rubber boot around the face of the piston takes any damage, rust can happen between the piston and bore. That'll seize it right up, which is easy to notice. A caliper with zero return can be harder to spot: the piston might not feel difficult to move, it just doesn't move back on its own.

You might want to make sure the pads slide freely in the channels of the mounting bracket, when the caliper is lifted off. They should move easily enough that the V springs on top really do push them away from the rotor when you let go. If things aren't that slippery, you can replace the little Teflon-ish coated spring steel pieces the pad 'ears' slide in; that's called the "fitting kit", has four pieces (enough to do both calipers) and is maybe $15-ish at the dealer.

-Chap

 

how can the caliper be frozen if it is sliding on the pins?

 

Start by raising the rear and doing the spin test?

 

How was the wear "uneven"? One pad more wear, or beveled wear?

 

Were you watching that process? Was he using the proper tool to rotate the rear pistons as he pushed them back in?

There is a spring clutch built into the caliper that is supposed to allow the internal threaded post to spin only one way (so the piston can adjust outward as pads wear, but cannot be pushed inward; the parking brake depends on that). If he was trying to mash the pistons straight back, to even succeed at all (let alone in 7 or 8 minutes) would only be by overpowering that spring clutch. That would take determination. New calipers might be on the horizon.

Slidability of the pins matters not, if the piston's not free to retract when the brakes are released.

-Chap

 

If you mean this pin:



That would be testament to the piston spoke pattern being misaligned, the raised spoke riding up on the pin, causing uneven wear. It's paramount to have the pin in between the raised spokes, and well seated there, so it doesn't slip and ride up on the spoke with parking brake application.

 

Check the pin too, if the top face of it looks bevelled/chewed, would be a clue that the spoke pattern rotated. I haven't got my head around it completely myself, but my thinking is that when you apply parking brake, it will rotate the piston if that pin isn't solidly locked.

I'm not saying this is for certain your problem, but I know from personal DIY experience, this is one thing that can happen.

One more symptom: the inside face of the rotor will be roughly 50 percent rusty, due to near zero contact.

 

Oh, good. I just had to ask, because with some mechanics, you just never know. And if you heard a mechanic say the rear pistons were really hard to force back, and he wasn't using a rotating tool, well, they would be really hard that way, and even if he succeeded (especially if he succeeded!), I'd have my concerns about the caliper.

But from your report it sounds like he knew to rotate it.

Remember that was in the context of my asking how he forced the piston back in.

But it does sound as if the pistons did not go back easily. You want them to move easily, because that's the only way the brakes let go. Applying the brakes isn't the problem, you've got 2000-psi-and-up brake fluid going back there so those pistons will extend out. But when you let go of the pedal, there's nothing but the rubber piston seals and those dinky V springs on top of the pads to make those brakes let go.

I gave some info in post #8 on how you can test the calipers you have and make sure they let go appropriately. I'm not an advocate of replacing things unnecessarily, so if you can test yours and get them to pass, that's great in my book. If they don't pass because the pistons don't move easily, yes, you can take them apart to find out why, if you're comfortable with that. You can get the rubber kit from Toyota in advance, it's cheap and has all the rubber parts (plus miscellaneous metal clips, etc.) to do both rear calipers. It doesn't include new pistons; if you find those to be non-salvageable, new pistons they sell a la carte.

Probably, at the very least, you would do well to replace the rubber piston seals (these are the square-cut ones inside the bore, only visible with the pistons out, not the visible rubber boot). Even if they were not the problem originally, if the brakes have been dragging for long, they've been very hot, and those seals are now probably cooked. And it's the exact way those seals twist and spring back that's needed to make your brakes let go.

I'm not certain that all the internal parts of the rear calipers are available over the counter. I seem to recall that the rubber kit did not include the seal where the parking brake shaft goes in from behind (or, at least, didn't seem to, in the diagram I was looking at). So be sure you know what replacements you have and what you don't, before going too wild with disassembly.

There's kind of a long-running controversy over whether to open the bleed screw when forcing the piston back. My inclination is to do so, which can make it easier to push the piston back, and avoids pushing a bunch of fluid from your brake line backwards through the brake actuator valves. In the end you have to add some fluid to make up for what came out, which means you've removed some old, dead-ended fluid from the lines, and put some nice fresh fluid in up front.

Some people never bother doing that, and just happily force the fluid backwards, and never have a problem. On the other hand, if a problem were going to result, it might well manifest as dragging brakes (if, say, a piece of grot were forced back into an actuator valve, preventing it from completely closing). I wouldn't worry about that, unless you end up with a new, or newly overhauled and tested, caliper, and it still drags anyway.

-Chap