Front caliper seized, how to rebuild master cylinder?

Discussion in 'The Garage' started by KennyFSU, Sep 25, 2019.

  1. lnewqban

    lnewqban Ninjetter

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    Before connecting the MC to the hose and caliper, did you prime the MC and made it pump like shown in the video above?
    #41
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  2. KennyFSU

    KennyFSU Look ma, no hands

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    Yep I did that but only until I started seeing some fluid squirt out. Should I go back and bleed the mc more?
    #42
  3. HapHazard

    HapHazard Waiting for Gudenov

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    Yes. You should purge as much of the air out of the master as possible.
    Keep goin' until it doesn't improve.
    #43
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  4. lnewqban

    lnewqban Ninjetter

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    One full stroke of the handle should push out relatively abundant fluid under pressure.
    Now we know that the MC is more or less working, although it could contain some air still.
    The spring inside the MC pushes its piston back, which sucks some of that mass of fluid back and retracts the caliper's pistons.
    That can happen only because the fluid is not compressible or expandable and acts like a "shapeable solid" inside the system.

    Removing all the air from MC, hose and caliper is key in achieving a solid mass of fluid that moves back and forth between MC and caliper.
    It is equally important to prevent air from leaking in when the fluid is sucked back into the MC.
    A common source of air leaking in is the thread of the bleeder (while it is open).

    #44
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  5. KennyFSU

    KennyFSU Look ma, no hands

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    I bled the mc by taking off the banjo bolt and holding my thumb over the hole and pressing the lever a few times. I could feel pressure building up so I would remove my thumb and allow the air and fluid to seep out.

    My question is during the next step do I plug the hole again and keep pumping? When I release my thumb should the lever be pressed or open?
    #45
  6. lnewqban

    lnewqban Ninjetter

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    Where in Florida are you? Maybe I could stop by and help you.
    I believe that it does not matter much: personally, I connect the hose to the MC and point it upwards, placing the open end of the hose higher than the cup.
    The idea is that the back and forth movement of the plunge, combined with presence of fluid at both ends (outlet and cup) that blocks air from leaking in, remove all air from the plunge cavity and cup.
    #46
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  7. Mercury264

    Mercury264 Once you go Triple...

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    I have had issues with trying to bleed a front brake with the lid off the MC. As soon as I put the top back on (plenty of fluid) it bleeds. Never been able to figure out why it does that but it's happened a couple times now so I just go with it.
    #47
  8. KennyFSU

    KennyFSU Look ma, no hands

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    I’m in Tampa. I’ll try that as well. Thanks.
    #48
  9. H96669

    H96669 A proud pragmatist.

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    Go find a syringue:

    #49
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  10. HapHazard

    HapHazard Waiting for Gudenov

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    ^Good suggestion. Sometimes, reverse bleeding is the only way to eliminate all of the trapped air in a brake system, and is nearly always the only way to easily bleed a hydraulic clutch.
    (Hearty, masochistic souls tempted to correct me about clutch bleeding, shaddap :lol3)
    #50
  11. Wentwest

    Wentwest How's that work?

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    If you're sure the caliper is bled of air then I found I could get air out by leaving the lid off the MC and watching the fluid as I gently sort of rattled the lever against the plunger. I wasn't pumping it, just lightly tapping on it with my fingertips so it was vibrating the plunger. A string of tiny bubbles came up through the relief hole, so I kept doing it until the bubbles stopped, waited a few minutes and did it again, until no more bubbles at all. It worked. My fantasy to explain it was that those bubbles were stuck to the inside walls of the line and the tiny shaking knocked them loose.

    I sounds nuts, but it worked.
    #51
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  12. KennyFSU

    KennyFSU Look ma, no hands

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    IMG_9553.JPG
    IMG_9549.JPG

    Tried reverse bleeding today and found some interesting info. The mc is actually blocking fluid from filling the reservoir (I unscrewed the brake line and was able to reverse bleed it right to the union bolt).

    So I took my mc apart again, do the cups look right? One doesn’t...
    #52
  13. lnewqban

    lnewqban Ninjetter

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    Something is blocking the flow of brake fluid back into the cup when there is no pressure on the hand lever, either the seal (B in the picture) is not fully uncovering the hole (C in the picture) when pushed all way back by the spring or there is an obstruction in there.
    Seal A seems to be damaged, but that one does not hold much pressure, it only stops fluid from leaking back towards the lever.
    Perhaps the internal surface of the cylinder is not smooth and has partially damaged the lip of the A seal.

    [​IMG]
    #53
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  14. lnewqban

    lnewqban Ninjetter

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    Also, I can't help you this weekend, I live too far from you, sorry.

    #54
  15. KennyFSU

    KennyFSU Look ma, no hands

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    I can get pressure to build up when I disconnect the brake line and bled the mc alone. The cylinder looked nice and shiny when I had the piston out. I used a pick and cleaned out the holes best I could.

    When I was reverse bleeding, no fluid was able to get back up to the mc whether the lever was pressed or not.

    The caliper is the only thing I haven’t messed with myself, a shop had rebuilt it and replaced the pads.
    #55
  16. lnewqban

    lnewqban Ninjetter

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    Next, I would reverse bleeding with the master connected but with no ring clip, moving the plunge out a little more than normal, in such way that the seal fully uncovers the compensating hole.
    That would hopefully tell either the compensating port or the seal are the obstruction to back flow.

    Is the cover of the cup properly vented?
    The big seal under the cover of the cup moves up and down when brake is manipulated and atmospheric air needs to flow in and out.
    I would also test reverse bleeding with the cup being open.
    #56
  17. HapHazard

    HapHazard Waiting for Gudenov

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    Sounds like this is the original problem - fluid can't return to the master.
    I've seen this on cars where the brake rod is out of adjustment (too long, and not allowing the hole to open) - does this master allow a travel adjustment?
    I've also had the dragging brake problem from deteriorated rubber brake lines.
    It's one of these two things.

    I think.
    #57
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  18. lnewqban

    lnewqban Ninjetter

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    [​IMG]
    #58
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  19. KennyFSU

    KennyFSU Look ma, no hands

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    I installed everything back together the way it was. No travel adjustment from what I can see. It’s weird because I’m able to bleed the mc with the brake line removed but as soon as I hook it up the fluid doesn’t go anywhere. I’m able to reverse bleed via the caliper and can get fluid to all the way to the Union bolt.
    #59
  20. markk53

    markk53 jack of all trades...

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    FWIW I had a can of carb cleaner I'd used to clean a carb, so I threw a stripped Yamaha XT225 master cylinder in it to clean it out. Worked well. Cleaned the caliper bits using some 1500 grit wet/dry sand paper, not coarse enough to cause any issues. Put them back together and bled them out with my trusty Harbor Freight vacuum bleeder. I also did that trick some recommend, pulling in the brake lever and securing it in place with a zip tie over night. Not sure if I'm right, but maybe the pressure forces air bubbles together and float up to the reservoir to bleed out at the master cylinder when the lever releases. But that's a guess. It helped a bit along with the vacuum bleed.
    #60
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