dual front disc caliper rebuild

Discussion in 'Airheads' started by LandLeftBehind, Sep 28, 2013.

  1. LandLeftBehind

    LandLeftBehind Been here awhile

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    Im about to attempt rebuilding both of my front brake calipers on my 91' R100rt.

    One disc is partially corroded where, Ive been told, the piston isnt making full contact with the disc. The rotor is a little out of true (0.007" - still within spec per Clymer), and this might be causing the problem, but I would like to rule out cheaper repairs prior to more expensive repairs. I figured I would do both calipers while Im at it, even though the other isnt showing signs of malfunction.

    The Clymer manual mentions nothing of the cross-over brake line connecting the calipers (the solid one that runs under the fender). Are there any precautions I should be aware of before detaching these lines from the calipers?
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  2. Plaka

    Plaka Brevis illi vita est

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    Corrosion. brake fluid Id hygroscopic. The water it attracts and dissolves corrodes the daylight out of things. As long as you are rebuilding the caliper, I would get generous with the heat when braking the lines loose. maybe start with a penetrant if you aren't going to get to it for a couple days.

    Cleanliness is critical. Hose out the hard line with brake cleaner then shoot some WD 40 in it and put in a clean plastic bag. Spray with brake cleaner again on re-assembly, dry with compresses air, anti-sieze on the threads and don't over tighten. Better it leaks and you snug it a bit more then you strip it and things get....unpleasant.

    A bent rotor can be straitened.

    Pistons don't contact disks, brake pads do.

    Corrosion is a chemical process. Do you mean erosion?

    You are not attempting anything. You have succeeded. Work backwards from there..

    P.S. Brake fluid is a fine paint stripper.
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  3. LandLeftBehind

    LandLeftBehind Been here awhile

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    Thanks for the tips. I was thinking of just leaving the lines in place, but I realize it might be better to remove them so they can be thorougly cleaned and protected from moisture.

    This is what I mean by corrosion:

    [​IMG]

    Several folks have told me this could be the result of a sticky piston.
    #3
  4. Plaka

    Plaka Brevis illi vita est

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    What do the faces of the pads look like for that wheel? You may have some stuff imbedded in them. Pics please.

    What does the other side of the disk look like? More pics.

    Is the shiny wear area deep or very shallow? Can you feel an edge with your fingernail between the shiny and dull areas? No pics of feeling disk. imagination will suffice.

    Look at the back of that pad. See the thick steel plate? it rides on two guide rods. What condition are those in? Pics.
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  5. disston

    disston ShadeTreeExpert

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    I would remove the caliper on the side that is showing the uneven wear pattern and clean it out. Take the piston out and clean it. Examine the insides for dirt and rust, pitting. And put it back together. Do the same thing on the other side. Install with new pads.

    If there is any leaking of fluids or the pistons are badly pitted then get the rebuild kits.

    The kits should be available with or with out pistons. Do not buy the kits until you know which kits are needed. If you buy the kits with out pistons you can not later add pistons. Pistons are available only in kits with seals.

    edit; What I have posted above is the story on Ate brakes. I think you have Brembo's? The story about parts and what is available may be different. You can still do the cleaning before you buy a bunch of parts.
    #5
  6. LandLeftBehind

    LandLeftBehind Been here awhile

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    This is the other side of the rotor (taken from the left side of the bike).

    [​IMG]

    Notice how the wear is on the opposite side of the rotor relative to the other side.

    [​IMG]

    The corrosion doesnt catch my finger nail, but it sounds rougher as I scrape my finger nail across it.

    Here are the pads:

    [​IMG]

    [​IMG]

    They appear to be worn at a slight angle, if I get the rotor straightened should i not reinstall these?

    The lockpins seem to be in fair condition, with only a bit of rust. They came out easily enough.
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  7. kaput13

    kaput13 gasoholic

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    The only precaution that you need to be aware of is - as already mentioned - the corrosive properties of brake fluid. Just take precautions in not allowing fluid to mar paint etc.

    You really need to rebuild these calipers. It would be foolish not to. Easy to do. Just buy the right kit. Seals are easy to remove and install. The only difficulty is in removing the piston. Don't settle for vice grips or some other makeshift solution to dislodge it but use air with a towel or piece of wood to catch the bullet.
    #7
  8. Plaka

    Plaka Brevis illi vita est

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    I'm seeing full contact across the pad, but not full contact across the rotor. Suggests they are the wrong pads. In fact, I'd bet a chocolate bar on it.

    if they are worn at an angle you can't re-use them any way. So you need new pads. I would get those and make very,very sure they are the correct pads for that bike. Put them in and begin the break in per the manufacturer. But just ride 25 miles or so. Then look at the contact pattern on the disk. You can put some radial lines on the disk with a sharpie, and rust ride around the block, braking as hard as possible. You will see how the lines wear off.

    Do clean the disk well first with a scotchbrite pad and some alcohol. Scrub so all dirt is off.

    When buying pads remember you have stainless disks. I would check out EBC pads. Decent price.


    The pistons have tremendous pressure behind them. They won't stick going out. if there is a real problem, they will stick on the retract. They only move a very small amount---they should be riding the disk lightly. But they should not be dragging and a piston problem will cause that.

    Things like rusty and pitted piston bores will chew up the seals. The pistons will leak.




    if your new pads don't give a good contact pattern, you can use them (because they are still flat) to check for alignment issues. if the entire caliper were tipped, the contact would be at the inside on one side and the out side on the other. not what you have. if the caliper was twisted (or the entire fork lower) then you would see a heel/toe wear pattern on the pads.

    You can also rotate the pistons. leave that check for after you do the new pads check.

    You can also check for caliper distortion (although I would expect leaking at the seam between the caliper halves)

    If you have a micrometer, I would check the disc thickness at the center and at the rim.
    #8
  9. LandLeftBehind

    LandLeftBehind Been here awhile

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    I really doubt they are the wrong pads because I never changed them, and this wasnt an issue until an accident I had last summer.

    Because im almost positive the bent rotor is the source of my problems, Im considering just waiting to send the wheel/rotors for straightening prior to working on the brakes. That way I can also rebuild the forks when I take the brakes apart (I was having some fluid leakage the previous summer).
    #9
  10. supershaft

    supershaft because I can

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    Something is seriously not right. What would take more and better pictures of the rotors, forks, calipers, ALL the parts holding in the pads and a description of how they were holding the pads in, wheel spacers and whatever else might not be right. I suspect the pads were not in place correctly for some reason?

    Straighten the rotor? Who? I would just get a new one unless the picture unfolds somehow a lot better than it looks now.

    There is no reason to flush the lines with anything other than brake fluid. There are GOOD reasons not to. Good brake practice is to not let anything but brake fluid and brake cleaner close to your setup. No need for anti-sieze or anything else. It contaminates the brake fluid. The thing you really need is the right tool which no one has mentioned: Line wrenches! Regular open end wrenches will distort the line nut and then it will leak no matter how much more you tighten it and distort it! I am hoping you are keeping your waders on around here!

    Once again, the condition of the piston bores does not matter. The piston seal is stationary. It does not move in the bore and the pistons themselves do not even come close to touching the bore. Corrosion on the pistons matters little too unless it is where the seal rides on the piston during the life of a pad.

    Pistons can and do stick in place and won't come out but retracting is a much more common problem.
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  11. Plaka

    Plaka Brevis illi vita est

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    sounds like a good plan. Except rebuilding the brakes. Do everything else first. You don't want to go into them unless you know positively they are bad. I never go into a brake cylinder before it dies. That means badly cracked rubber on the outside or leekage from around the piston seal. You see it (loss of fluid) before you are in brake failure land. but fix it as soon as you see it. With the pads out, very gently squeeze the lever to extrude the pistons slightly. Go too far and you can trash the seals. Then try squeezing the pistons back by hand. Two hands. See how they feel. it's not unusual to use a c-clamp to compress pistons and you can try that if they don't move by hand. but just see if they are smooth,.

    you still need to replace the pads tho'. Don't try grinding them flat to milk more life out of them. Spend the fat insurance check from the crash on nice new pads. if you want more serious brakes fit iron rotors. but those brakes should be plenty if they are anything like my old K bike. Wish I had a set.


    The typical symptom of a warped rotor is chattering or pulsing. You will see a wear paterns that's unever around the disk, not from the inside to the outside.


    possibly the calipers are spreading? They would spread more on the outside than the inside so less braking at the rim. But they would have to be permanently distorted to get what you are seeing. Check with the new pads. put them in, squeeze lightly and see if they are touching towards the inside of the disk but not near the rim. Then rotate the pistons 180 (both) and check again.
    #11
  12. LandLeftBehind

    LandLeftBehind Been here awhile

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    I cleaned up the pins yesterday, brushing all the dust off of them, and geeze - they are chewed up - one more than the other. They are still straight, and fit into the calipers like they should, they are just really pitted. Not with rust either - just looks like a surgeon in the civil war gave them to his patients to bite down on while he amputated limbs.

    I wonder if that is a symptom or a cause. Eitherway they are going to be replaced.
    #12
  13. H96669

    H96669 A proud pragmatist.

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    Are you checking the rotors "as found" on the wheel??? I had one on a GS that showed really close to the max deflection allowed. Removed it and carefully cleaned the mounting surfaces, back on the wheel and then well within specs. Had to check it carefully as I sold it later, did not want a new owner to experience pulsing.:eek1
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  14. supershaft

    supershaft because I can

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    Something is not right. I hope you have someone with experience look at that. Or at least more pictures here so we can figure out what is going on. That stuff can hurt you! :D
    #14
  15. LandLeftBehind

    LandLeftBehind Been here awhile

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    Thanks for your help SS. My hunch is that this all is due to a warped rotor that could have happened last summer when I met a deer on the highway. The brake pulsing is quite evident, especially during moderate/hard braking. Your opinion is very much appreciated though.

    Here are the pins:

    [​IMG]

    [​IMG]

    Want to see anything else specifically?

    H96669 - Im not sure I catch you - are you saying a dirty rotor can cause pulsing? I havent cleaned mine thorougly ever...
    #15
  16. boxerboy81

    boxerboy81 Stay Horizontal

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    Yes it can. I use brake cleaner to lightly wipe my discs over occasionally. Sometimes even a very fine wet and dry or a scotchbrite. My bike is used to commute on suburban roads and I feel they build up with grime due to a lack of regular hard braking. The wipe over certainly removes a mild pulsation.
    I find a good solid ride in the mountains where I'm hard on the brakes will clean them up too.
    #16
  17. Plaka

    Plaka Brevis illi vita est

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    Looks like rust pitting. Replace all 4. In a powerty rider situation you can spin them up in a drill, scotchbrite/sandpaper them (wear a leather glove!) and run them. The rusty part is under the clips more then the pads.


    Someone may have changed the pads bu putting a vice grips on the calper to tap them out rather than correctly using a punch through the wheel. Can chew them up.

    I'd be interested in a silhouette shot of the new pads installed and under very light pressure. You are looking for daylight at one part but not the other. Shouldn't be any.

    Do you have some bore gauges and a micrometer?
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  18. Plaka

    Plaka Brevis illi vita est

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    Cleaning up the brakes with the pads means the crap gets imbedded in the pads. Your scotchbright work sounds better. Running scotch prep disks in a drill, not a die grinder, works well too. Not overly aggressive with light pressure and very fast. hard to get to the inside tho'
    #18
  19. H96669

    H96669 A proud pragmatist.

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    I should dig up my pics that show corrosion and maybe bumps on the mounting surfaces, rotor and wheel. That can be enough to make them pulse.If you look at the specs for max deflection, BMW doesn't allow much before they say "replace".

    I built a jig to check wheel/rotors deflection and found some on the GS rotor. Enough deflection to make that rotor useless and I wanted to sell it. Sold and I told the owner that I did check it properly and how I did it.

    Removed the rotor, used the little file to indentify the raises. Cleaned everything and brought the surfaces back to flat. Remounted the rotor and back to well within specs.

    Rusty pins....they all do that. Well maybe not "all of them" but for the ones of us riding the salty roads or salty gravel, always been a problem. At least yours came out easy, not always the case when the circlips corrodes in the caliper.:eek1

    You can check with Motobins they sell them pins in Stainless.
    #19
  20. LandLeftBehind

    LandLeftBehind Been here awhile

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    I bought a micrometer today and measured the dics. Turns out im pretty close to the wear limit of .14" - the warped disc averages at about 0.158 the straight disc is about .152"

    How much "life" does .01" translate into? A few thousand miles? Im not familiar with how discs wear with use so getting a sense of that would help me determine if they or worth straightening or if I should just get new ones.
    #20