Front caliper seized, how to rebuild master cylinder?

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

  1. Beezer

    Beezer Long timer Supporter

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    won't be any. if the lines fail it will all be downstream
    #21
  2. Lowndes

    Lowndes Adventurer

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    Well, the good news is the DOT4 has been changed recently.

    It's time for a good overhaul of the brake system, including new lines, thorough cleaning of the calipers and MC, replacement of any worn or corroded parts, new DOT4 and bleeding. Maybe new brake pads, too.
    #22
  3. baloneyskin daddy

    baloneyskin daddy bikaholic Super Supporter

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    You won't see it squirt up because the bleed hole is under that little metal flap in the bottom of the master cyl. Just pop it off and run a small wire thru it if you see no return pressure.
    #23
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  4. kenstone

    kenstone worn out

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    Yes that, and a there's lot of smooge in the bottom of the reservoir...
    An indication the hose has become degraded and is falling apart.
    Yes, smooge from the hose getting back into the reservoir via the bleed hole.
    :lol3
    #24
  5. squish

    squish Out of the office.

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    Here's the deal.
    I got a bike dirt cheap because of this very issue.
    Rebuilt the calipers, new hoses, rebuilt the master cylinder, went over the system a few times, until I found the issue.

    The body of the master cylinder is made of aluminum or some other cast pot metal alloy,
    That reacts with the water that brake fluid absorbs.
    That then fills in the small return port into the master cylinder, and the reservoir.

    When brake fluid heats up it expands, if there isn't a way for that fluid to make it back into the reservoir,

    Clean the little port or passage way and it should all work,
    However it sometimes hard to find it, especially when the reservoir is cast into the master cylinder body.

    If you can't get it clean it might be easier just to buy a new or at least new to you master cylinder.

    That's what solved the problem on my bike, with the new master cylinder it worked great.
    #25
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  6. KennyFSU

    KennyFSU Look ma, no hands

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    I have a mc rebuild kit and a new brake hose coming (caliper was already rebuilt), I'm hoping that will fix it!
    #26
  7. kenstone

    kenstone worn out

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    Just me, but I'd try and find the problem before throwing parts at it and hoping that will fix it.:dunno
    You are going to have to take it apart to install the new parts anyway, why not do that now, and look around for what's going on in there.
    Like the bleed port blockage that several people besides me have mentioned, and you have ignored.
    I have brought a few old/neglected/left out to rot MC back from the dead, have yet to put new hoses or "kits" on them, just a simple clean and new fluid, and still ride them today.
    Maybe I need to buy a lottery ticket.
    An '83, different tank original had rotted out, original brake lines:
    FrtLsS.jpg
    #27
  8. squish

    squish Out of the office.

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    That's a start but typically a master cylinder rebuild kit is just the piston and seals and related clips and springs.
    If it's a blockage in the reservoir return port, that rebuild kit will not fix it. You have to clean out that port
    There's a dental tool that I have found to be moderately effective for that job.
    But it depends on how deep the cylinder bore is, and where the return port is located in that bore.
    This is if you can't get to it from under the brake fluid reservoir and only have access to it from the cylinder bore side.

    The tool, it is for measuring the gum pocket depths and it looks something like this
    [​IMG]

    You can find them at tool swap meets or at those tool shops with all the knock off Chinese tools that every large city seems to have.
    Or you can ask your dentist if they happen to have any that are past their life cycle and they are tossing them out.
    Look for the smallest size probe you can find.

    This tool is also good at getting the gunk that builds up in the caliper seal ring lands in the caliper body.
    #28
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  9. Wentwest

    Wentwest How's that work?

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    accupuncture needles are also good for these tiny jobs, and they are cheap.
    #29
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  10. squish

    squish Out of the office.

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    The big challenge for this job is that this passageway is in the cylinder and you need something with a bent tip to get to it, if you can't get to the hole from above.

    otherwise acupuncture needles would be really effective for this.
    #30
  11. Lowndes

    Lowndes Adventurer

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    "I have a mc rebuild kit and a new brake hose coming (caliper was already rebuilt), I'm hoping that will fix it!"

    Kenny,

    "Rebuilt" could mean several things. I don't know about other manufacturers but BMW is emphatic on NOT splitting the calipers, ever. (liability issues, I presume). Even the Clymers says "do not separate the caliper halves...", so if your caliper was not split by your shop you could still have junk in the galleries. You might ask them if they split the calipers in the rebuild.

    I split my BMW calipers (Brembo) because they are MY freakin' calipers, and I'm glad I did. There was NO possible way to clear the junk causing the lockup and thoroughly clean it otherwise. They are stoopid simple, four bolts, and an O-ring but the attorneys say no.
    #31
  12. kruzuki

    kruzuki Gear in the Machine

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    Wow, I remember good old Dan's MC page!
    Probably one of the oldest motorcycle sites on the web. But he still gives good fundamental advice.
    #32
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  13. KennyFSU

    KennyFSU Look ma, no hands

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    View attachment 1926756

    Just rebuilt the MC, brand new brake line, and fresh dot3.

    Only issue now is I can’t seem to bleed the brakes. Opened the mc cap, put a clear tube on the caliper nipple, several squeezes of the lever, hold, loosen nipple and allow fluid and air to escape. Repeated this process several times and not much air escaped and the lever still has no resistance.

    I even bled the mc by removing the banjo bolt and placing my finger over the hold and pumped the lever.

    Any ideas?

    Attached Files:

    #33
  14. lnewqban

    lnewqban Ninjetter

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    Could you disconnect the hose from the MC and verify that the new internal parts are properly pressure-pumping fluid?
    #34
  15. KennyFSU

    KennyFSU Look ma, no hands

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    Yeah I did that and felt suction when pumping the lever.
    #35
  16. lnewqban

    lnewqban Ninjetter

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    You should feel pressure out.
    Perhaps the pistons were installed backwards?

    How to test the MC:



    Copied from the link that I posted earlier:

    "So, you pump and you pump... nothing. Now what?
    1. Try bleeding the air from every fitting
      on the hose, such as where the hose attaches to the master cyl. and anywhere else that there is a fitting.​
    2. Try filling the system from the bleeder screw with an air pump and bleeder jar.
    3. Tap the fittings lightly with a hammer to jar the air bubbles loose.
    4. Let things set overnight, then do everything over again.
    5. Be patient, keep bleeding the system... this may take quite a while.
    6. On at least one China clone bike I had to take the master and slave cylinders off the bike and turn them every which way while pumping the master. I left the caliper loose on the rotor so the piston didn't pop out. It took forever but it finally pumped up."
    #36
  17. KennyFSU

    KennyFSU Look ma, no hands

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    Sorry I misspoke, there is pressure out.

    Thanks for the vid, I’ll review the steps again.
    #37
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  18. MotoChris521

    MotoChris521 expert apprentice

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    Reverse bleed. look it up.
    #38
  19. KennyFSU

    KennyFSU Look ma, no hands

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    I saw that in my Clymers manual and will give it a shot.
    #39
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  20. KennyFSU

    KennyFSU Look ma, no hands

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    I'm at my wits end here.

    Even with the caliper nipple closed and full brake fluid in the mc I cannot build up any pressure. When trying to bleed the system I did notice some dark swirls starting to appear in the mc (did not see any bubbles or squirting of brake fluid when pumping the lever though).

    I can see a small amount of fluid/air coming out of the caliper nipple hose but nothing significant.
    #40