Did you try bleeding that caliper?You could try removing the caliper,then spraying well with some brake cleaner,and pump the brake.Could be some brake material caught up in there some way.See if that frees it up.You wouldn't have to pump the lever really.Just see if you can push the two sides in(one side at a time on the caliper).They should come out equally(as you know).

* Last updated by: Grn14 on 8/16/2017 @ 4:22 PM *

No no experience with either questions but here's my thoughts:

And if any of you have experience, how tough is the paint on the caliper?

Well, that I can tell you is very resistant to brake fluid. A good wipe down with a dry rag should be fine even if you don't get to it immediately. I wouldn't leave any fluid on any other painted or bare plastic surface though. Rinse with water and wipe dry right away.

I've been told NOT to pull the brake lever with the calipers off or the pistons could get pushed right out. Maybe the pads would stop them but with no pads in between and no rotor, that must leave enough room for the pistons to pass right through. Yes, I am sure you will have a few oz of fluid trickle out as soon as they drop out.

If you had a bore/piston diameter problem, that seems very unlikely. How would one bike out of thousands get bored too small? I doubt that is it.

As far as your question regarding reason for the uneven wear, I think it would be much more likely the side with more wear is dragging because the pistons do not move as freely as they should. Brake presssure is pretty strong. The problme is not usually that the pistons will not extend. The pistons have no way of retracting except by rubbing and getting nudged back in by contact with the rotor. That's why they squeal sometimes when you roll the bike---the pads are always kissing the rotor surface. Yours are playing smashmouth on one side so that's why they are wearing faster. They don't retract as easily as they should.

JM .02

* Last updated by: Rook on 8/16/2017 @ 8:32 PM *

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I doubt there would be as much of a "spray" of brake fluid, but there will definitely be a lot fluid spilling/flowing out of the caliper after you eject the piston.

I had to replace the seals & boots on a stuck piston on the caliper of one of my cars, thankfully I do have compressed air but using the braking system might have been more gentle (using 90-120PSI to break a piston loose isn't fun at all).

You will need a couple of C-clamps and a piece of wood or something to hold 3 of the 4 pistons in place. You will then use the brake lever to eject 1 piston at a time to replace the seals. The pistons (at least in my car) aren't nearly as long as you would think but you will need to be careful with the positioning of the clamps/wood to hold the other pistons.

As Grn14 mentioned, bleed BOTH calipers first, if you see bubbles of air coming out, see if that fixes the problem first. As for which seals, the ones with little to no wear are the ones with the problem since they aren't pushing against the rotors and the other pads that are working are doing all of the work. On my car the inside pads were at 80% and the outside pads were down to 5%, the inside pistons were seized and needed to be fixed.

If it is the seals, it was actually really easy to replace the seals and dust boots (don't think the bike has dust boots now that I think of it). Pull off the old dust seal (if there is one) then pull off the old seal out of the groove, put the new seal in (use some clean brake fluid to lube the seal), make sure the caliper bore is CLEAN and free of defects (cuts, bumps, etc) press the piston with the new seal back in, then press the dust boot in making sure it fully seats. After you've done all of the pistons, bleed the system.

A couple of things that should be obvious but just in case, wear safety glasses!! remove the brake pads, dismount the caliper, but don't let it hang by the hose (use a piece of wire to hold it), cover the rotors and anything you don't want brake fluid on.

Theory goes like this:
Quad ring = When piston is pushed, a 4 sided rubber ring sits square in the caliper groove.
Pressure applied = Piston moves forward, quad lifts out of groove, water (vapor) condenses between ring and groove.
Chemical reaction = The water crystallizes, becomes hard, causes quad to hang up or squeeze around piston.
Memory = Where the quad would return back to square as it pulls the piston back with it.
Pad wear = Now becomes uneven: as the chemRe settles in the groove from neglect of purging old fluid which is leftover past the 2 year interval.

Shims have to be in place so as to move all 4 pistons out evenly or you lose the pressure to the whole system. How do you remove the other pistons if you just work getting out the one side.

Cleaning:
Do not sand piston or caliper finish.
Use paper towels and say WD40 as a cutting agent.
Clean with brake clean to remove any oil and crystal remains.
Let dry, then note white chemRe still in the quad groove. All must be removed with scribe or dentist tool like pick.
Replace new quad rings with new.

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Thanks Hub...hoping you'd come through with an actual procedure for this...

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You will need a couple of C-clamps and a piece of wood or something to hold 3 of the 4 pistons in place. You will then use the brake lever to eject 1 piston at a time to replace the seals.

That sounds much easier than forcing compressed air in there. Air compresses a lot more than brake fluid so I imagine hooking up a compressor to a banjo bolt or something would be a lot less predictable.

...but wait a minute--once KAK pops the first piston out, pretty much every bit of fluid is going to drain out right behind it. Seems like he's going to have to do all of them at once or at least one side at a time.

I think the two that are easiest are going to pop out all the way. Then KAK will need to pull the others out with a rag. I'd go slow on the lever and watch what is happening to the pistons. Maybe do one caliper then refill the system and bleed it before doing the other caliper.

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So I don't know how each pad could be "tilted" .01" top to bottom.

Think of the pads grabbing the turning rotor. The pads are just hanging loose on a simple pin. They're probably going to grab more at the leading edge and the trailing edge is going to tend to spread apart just a bit. Yeah, the pistons are hydraulic and they should move exactly perpendicular to the caliper but the pads are comparatively crude mechanical parts. Lots of play. Lots of opportunity to twist just a bit. My pads were a bit thicker on one edge too. It seemed normal to me. All pads followed a consistent pattern. I think the top pads were worn less than the bottom pads too. Again, leading and trailing. I wouldn't worry about that. You might fix your pistons just by doing what you're doing, working them in and out. Take the pads out and stick a piece of plywood in there. clamp them down and pry them back open a few times. That might be all it needs. You don't really need to lube. The brake fluid lubes the pistons.

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Unless the pad wear is really excessive, such as how my car was (80% on 1 side 5% on the other) I wouldn't worry about minor differences. With this hydraulic system, when you apply pressure the fluid is going to move to the path of least resistance first.

For the sake of the conversation, assume that all 4 pistons have the exact same resistance for the piston itself, the piston closest to the master cylinder is going to have the least overall resistance as it has the least amount of fluid/hose to go through, so it would hit the rotor first. Followed by the next piston, then the first piston on the other side of the caliper then the last piston. This would all happen VERY fast but would technically result in slightly different rates of contact with the rotor.

As you observed, when you remove the caliper so the pistons can't push on the pads/rotors when you push 1 piston back in, the path of least resistance is typically the next closest piston (if all of them have the same resistance).

A trick to pushing the pistons back in is to loosen the cap on the reservoir to reduce the resistance, although I never found that made a lot of difference. Would work on bikes as well, but for cars, specifically with integrated emergency brakes in the rear brake caliper, you can crack the bleeder open slightly as you drive the piston back in. This will relieve a lot of the pressure making the piston very easy to push back in at the cost of a little bit of brake fluid (make sure no air gets back into the system or bleed it after doing this).

Do them right, and completely. The caliper pistons develop a line of debris at the seal lip, when you start pushing the pistons back into the caliper where does that line of crap go? Order the small O-rings that go on the caliper body bolts along with all of the other seals and do a complete rebuild. Remove the calipers and put them on your work bench, split the halves, push the pistons out and completely cleans the pistons/bores/seal grooves. Do not use anything abrasive on the pistons or you'll remove the very thin coating, use a pick as suggested earlier to remove all of the built up crud in the seal grooves. Install the new seals, lube the pistons and push into the bores, bolt the caliper halves together using new seals, reinstall the line and bolt to the fork. Bleed properly and you should be good to go.

part number 92049-1448

I misspoke on the "caliper body bolt", it's on the fluid transfer port from one half of the caliper to the other half.

* Last updated by: C14 on 8/19/2017 @ 5:00 PM *

I wouldn't worry too much. We're talking 6/100 of an inch. They make floating rotors for a reason but I doubt that small of a difference would make a noticeable improvement in braking.

* Last updated by: Rook on 9/1/2017 @ 12:51 AM *

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