Premise & Background: I have bought too many unnecessary parts over the years and have been trying to be more certain about what the cars need before I spend. Also, my 2010 Prius with about 310k miles --150k of which I put on myself -- has new tires. They're about ten weeks old and at most only have a few thousand miles. Everything was working, then my wife said she heard brake grinding a couple of times, after the car had been parked overnight. I pulled the tire and saw there was visible pad, so I ignored it for a few days and tried to blame road dust until the grinding became more frequent and then, there was a new rhythmic sound that you mostly heard when going slow. This prompted me to pull the brakes and I found that the pad on the back side of the front passenger side was unevenly worn. There was still some -- not much, but some -- pad visible from the top, but the bottom of the pad on the leading edge was worn past the ceramic down into the metal back. The rotor was also shot, so I replaced it and the pads on both sides. While the bracket was off, I pulled the slide pins and filled the holes with Brakleen a couple of times, took a wire brush to the pins and clips, washed them also with brake parts cleaner, then lubed them up with brake-grade silicon grease. After putting the car back together, the rhythmic noise was louder, more constant and more pronounced. (I'd describe the noise to be like you used to get from a bubble on a recap or when there's a big worn spot with the steel belts showing. It's more of a flap than a rattle.) I confirmed the noise was coming from the front passenger side, so I took it back apart after driving less than ten miles and switched the slider pins from one side to the other to see if the problem moved. It did not and though it's only visible if you're looking for it, I'd say the new pad is starting with the same wear pattern. My instinct is that I probably need a new caliper, but as I said, I'm trying to avoid buying unnecessary parts. Another factor is that though I don't know what the previous owner did, I haven't drained and replaced the brake fluid over the eight years and 150k I've owned the car. I've thought about trying it before I buy a caliper because it can't hurt anything, but I'm also tied of popping the tires on and off. Also, when I pressed the calipers for the pad change, the brake fluid reservoir was above "max". Suggestions would be appreciated. Anything else to test or try? Any thoughts? Thanks
You neglected to say if you tested the caliper slide, after reassembly? Was it moving side to side easily? Did you test the wheel bearing for wear? Almost everyone knows to always check the INSIDE pad (piston side), if you suspect the pads are getting thin. Piston side pad wears slightly faster, since that's where the braking power is first applied. The slide pins allows for equal distribution of force between the inside and outside pads. You'll need to pull those pads out and make sure the calipers moves freely, side to side, so the pads can distribute the piston force evenly between the inner and outer pads. Are you getting any brake peddle pulsation? .
Does the caliper piston move freely, and does it have the proper return? After my experience in the post linked above, I'd generally be inclined to restore (if that's needed) a caliper, if it's the original Toyota one to start with, rather than replace it, which is likely to be a downgrade. The rebuilding rubber kit is cheap, and has the parts to rebuild two calipers.
Though I've done lots of brakes on my own cars for decades, this is the first time it is something other than replacing the pads or the rotors, so I'm not sure what you mean by "caliper slide" or "side to side". Please bear with me, as I learn. The slide pins go in and out, and I did switch sides, but the sound didn't move. There might be a slight catch when you first pull a pin on the problematic side -- that's the tire I have off with the caliper hanging -- but I'd say the pins slide easily and if there is a "catch", it is only slight. When I slide the pins, I see nothing else moving on the bracket (with the pads out) and other than the piston, I don't see any obviously movable parts on the caliper. When all this started, when I first took everything apart to replace the rotor, I did have to use an 18mm box end to hold the pin while I took off the lower bolt, but that's really the only indication I've had of binding. Otherwise, I have pressed the piston in at least three times and I did leave it hanging after the pin switch, while I started the motor to see what happens. What I found was that piston kept incrementally going out every time the actuator activated without anyone touching the brake. I let it go maybe three or four times, so the piston was extended out probably two inches. The rubber looked fine and everything was dry as a bone, if that means anything, then I pressed it back in totally flat. (After my test drive on the new rotor with new pads, the piston was out a millimeter or so, but it pressed back) In answer to your other questions: I feel and have not felt anything in the pedal -- it's just a noise that you hear both in and out of the car -- and I did the 12 o'clock/6 o'clock thing and there was no movement, so I don't think it's a bearing. Thanks
I don't know if you intended to post a link, but I'm not seeing it. As far as I know, it is the original caliper and I have no reason to think it's not. As for the "above max", when my wife first told me about the grinding, I saw that the fluid was below minimum, so I poured a bit in to get it between min and max. I didn't max it out because I thought maybe I should flush the lines because it hasn't been done and I didn't want to waste fluid. After I pressed both sides back in, the fluid now registers above max, probably by the amount I put in. I've topped it a couple of times and have kept a pretty good check on it over the years.
Sorry, thanks. Fixed above. The caliper has a common single-piston design; the only piston presses on one of the pads, and the equal and opposite force slides the whole caliper body the other way, so the other pad experiences the same force. The pads are not supported by the caliper, but by the (stationary) caliper bracket. The 'ears' of the pads rest in channels in the bracket. Well, really, they rest in plated springy clips (the "fitting kit") that clip into the bracket channels. The caliper, supported by the bracket and slide pins, just sits over the pads, and squeezes them when applied. The caliper has to be freely movable side-to-side, which is the job of the slide pins. The pads also have to be freely movable side-to-side, which is the job of the dry Teflon-like factory coating on the "fitting kit" clips. (Any actual grease there would be a grit magnet and make things worse.) Sometimes if the bracket has rust, the channels for the "fitting kit" get a bit narrower from rust jacking, and the fitting kit clips get clamped down onto the pad ears too tightly for the pads to move easily. More often, the fitting-kit clips first lose their Teflon-like coating, and then start to get rusty, and the pads don't move easily. It is also possible for the caliper piston to become non-free-moving, especially if its rubber boot has taken some damage and allowed salty water in there. And it is possible for the piston to be quite free-moving, but simply not return by the 0.3 mm or so that it's supposed to when you release the brakes. That's the job of its square-cut rubber seal, as explained at the link I (eventually) supplied above. As you can see, there are a bunch of places easy movement has to be possible; it's a system that only works right when it all works right, and there's nothing very hard to check, but the smart money checks it all. In my experience, fitting-kit issues are most common, and slide-pin issues, for whatever reason, I only experience vicariously by reading PriusChat, where they seem to suffer from a bit of "we are the parts everyone has heard of, unlike all the other things worth checking" syndrome. But they are, after all, among the things worth checking. I don't really see a connection between the brake fluid level and whether it is or isn't advisable to rebuild a caliper. The Prius brake system confuses people because when the pump runs and pressurizes the accumulator, some fluid disappears from the reservoir and the level drops. The marks on the reservoir are intended to be read with the system "zeroed down", that is, with no pressure in the accumulator and all the fluid back in the reservoir. If somebody looks at the reservoir marks when the system isn't zeroed down, the level will look a little lower. If anybody ever topped off the reservoir when the system wasn't zeroed down, the level will look a little high with the system zeroed down and the piston pressed back. That's not uncommon at all (it's more like, people knowing the marks should be read zeroed down is what's uncommon).
The reply from @ChapmanF helped me understand what should be moving then after trying both sides with the pads in place -- the only movement like the thickness of a business card -- I reread your reply and saw that you said with the pads out. With the pads on the passenger (or problematic) side out, I can rock the caliper end out the full length of the sliding pin and I can pull it toward me enough that I'd call it bottoming out. With the pads uninstalled, I had someone push the brake and when that happened, the caliper moved as the piston extended, but it didn't go back. Also, though it didn't happen the first two or three times she pushed the brake, after those initial applications, brake fluid came pouring out of the piston. She also said that every time, it felt like the pedal was pushing back against her. After the experiment with the caliper in place, I removed the bolts and hung it, so I could see. I, then, had my daughter depress the brake and each time she pushed it, the piston extended further and when she released it, you could see it easing back a bit. It did not recede a lot, but it was enough to clearly see.. Anyone have any thoughts? My instinct is this is how they're supposed to work. Though, I'm not sure about the fluid or why I didn't see any fall to the ground when she first braked. Thanks
If I had known you were going to try that, I would have warned you against it. Yes, without the pads in place, the piston can be pushed right out and dump your brake fluid. In fail-safe mode, it may take several pumps of the pedal to accomplish that. If the system is powered up in normal mode, you may be able to blow it right out under accumulator pressure in a single step. You now have a caliper-reassembly project (being here already, I'd go ahead and get the rebuild rubber kit from the dealer and change those parts out), and then a brake refill and bleed project (using a suitable scan tool). If you were wanting to measure the piston return, here's a pic from the thread I linked to that explains how to do that: You can see the blocks put in place of the pads and rotor so the piston isn't ejected. As I said in the post explaining it, I use just gentle puffs of compressed air on the bench. An on-car piston return test isn't a complete impossibility, but has to be done very gently because the braking system develops enough force that you easily end up just measuring the elasticity of steel and aluminum instead of the piston return.
Edit: Never mind. I see. When I went to compress it, the piston fell out. It looks like I probably didn't need a new caliper until I had my daughter push the brakes while the caliper was hanging from the strut. Now it looks like a rebuild or the $80 part I had not wanted to buy. 2nd Edit: I'll read them again, but I did not initially see rebuild instructions in your previous link. I did find a video which I scrolled through... because it's almost dark, I'm going to pull the caliper and haven't actually looked at things, but is there a reason I need new rubber and can't just use the old parts?
