Front caliper seized, how to rebuild master cylinder?

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

  1. KennyFSU

    KennyFSU Look ma, no hands

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    I had the caliper rebuilt by a shop recently and the front brake was working fine until yesterday when it seized up again. Shop says the master cylinder needs to be rebuilt now so I bought a rebuild kit and plan on doing it myself.

    I've seen a few YouTube videos on other bikes and it seems pretty straight-forward but I was wondering if anyone had any other tips or resources they use to rebuild one for a '91 Intruder 750, thanks all.
    #1
  2. Ducky 149

    Ducky 149 Been here awhile

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    There is a really long snap ring plier that might be a MUST have tool depending on your M/Cyl. Look it up on EBay
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  3. KennyFSU

    KennyFSU Look ma, no hands

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    Good call, just ordered a pair from Amazon, thanks.
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  4. lnewqban

    lnewqban Ninjetter

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    General but good information here:
    http://www.dansmc.com/hydraulics.htm
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  5. JR356

    JR356 Long timer

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    Just be sure it is not a collapsing aged rubber brake line,as opposed to the MC.

    JR356
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  6. dddd

    dddd Long timer

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    forgive my possible ignorance but what does a seized caliper (pistons) have anything to do with the master cylinder 3 feet above....?
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  7. Lowndes

    Lowndes Adventurer

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    If your OEM type black rubber brake lines are 15 years old or older, they definitely need replacing as they are past their sell-by date. DOT4 eats paint, plastic, and those brake lines, it just takes a little longer. The front wheel on my NTM '99 BMW R1100S locked up while riding three years ago.

    After disassembling the front caliper I found decomposing brake line chunks in the galleries making a check valve, letting pressure into the pistons but not out. I replaced all the flexible brake lines and the hydraulic clutch lines with a kit from Spiegler. Galfer makes similar kits. These lines are the "braided stainless" with a clear or colored plastic coating and PTFE internal tube. It was about $200 then for the complete kit. Took maybe half a day to thoroughly clean the calipers and install and bleed everything. EZPZ if you know which way to turn a wrench.

    There seems to be two ways these old brake lines fail, one is the way mine failed (and I'd bet yours did, too), the other way as an aneurysm or big bulge and eventual burst leaving you with NO brake. It's usually the front brake lines that fail first because of the movement, steering and suspension, and the harder usage.
    #7
  8. Wentwest

    Wentwest How's that work?

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    The other thing I learned from rebuilding old neglected bikes is that there is a very tiny sort of jet in the master cylinder that is only completely exposed when the piston of the master is fully released, and it lets pressurized fluid flow back into the master. That tiny hole is often plugged up when the caliper seems to be seized. First step is to put a tube on the bleeder of the caliper and open that bleeder to see if there's pressure in the system. With the lever released there should be none at all. If there is the problem with the lines is possible, but so is a plugged up relief port that can be cleaned out with a needle.
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  9. JR356

    JR356 Long timer

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    Forgot to add,that if your shop did not identify this as a potential issue on an older bike,time to change mechanics.

    JR356
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  10. Beezer

    Beezer Long timer Supporter

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    like Lowndes said in post 7.... brake lines. it was pretty common on BMWs around the Y2K years. I lost brake, had a brake seize, and had rubber crumbs contaminate and kill the ABS all due to failing lines. sounds like what your problem is to me.
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  11. KennyFSU

    KennyFSU Look ma, no hands

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    Thanks for the tips, all.

    The front brake seems to work fine when left alone to cool down. After riding it for a few minutes and getting it warmed up it will seize on me again, any ideas??
    #11
  12. Lowndes

    Lowndes Adventurer

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    KennyFSU,

    If you have the clog from decomposing brake lines like I did, YES, it will continue and get worse. What I'm afraid youre are experiencing is the "check valve" type clog. It isn't a perfect check valve and will "leak down" over time, what you are calling "cool down". But as it gets used it will take longer and longer to leak down as more junk settles in there. And you are correct, it does cool down at the same time, too.

    This is the clog in my caliper, in the junction between the caliper halves (looks like a head and shoulders sticking out):

    [​IMG]

    There was also a chunk of the same junk in the bleed nipple port:

    [​IMG]
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  13. KennyFSU

    KennyFSU Look ma, no hands

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    Since my caliper was recently rebuilt do you think the clog could be in my MC? Someone said to check the two holes for clogs there as well. Or it could be bad brake lines. Thanks.
    #13
  14. Lowndes

    Lowndes Adventurer

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    YES, certainly check the MC. The port is usually visible the reservoir (if the reservoir is part of the lever/MC and not a remote type) as one or two VERY small holes on top of where the MC piston is.
    Remove the reservoir cover, make sure the fluid is full, and give the lever a good, quick pull. If it squirts DOT4 straight up in the air and all over everything you're in the right place. An old Indian trick is to drop a quarter, nickel, or a washer in the reservoir to cover the holes so the DOT4 doesn't squirt out.

    Take some pics and tell us what you find.

    ALWAYS keep a windex spray bottle and a few wet rags handy when fooling with DOT4. Water cuts it and it will save your paint, plastic, windshield, etc., if you get on it quickly.
    #14
  15. Beezer

    Beezer Long timer Supporter

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    take the line loose at the bottom and pump fluid into a container. if the line is failing you will probably see hunks of rubber
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  16. kenstone

    kenstone worn out

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    … and pushing the caliper piston/brake pad back (with bleeder closed) will put added pressure on that bleed hole and may dislodge anything blocking it.
    This can be done a C-clamp as the caliper slides on pins and is self centering.

    If that doesn't work take a single copper strand of wire from a multi-strand electrical wire and poke it thru that bleed hole.
    This bleed hole can be found in the bottom of the reservoir, towards the back directly over the centerline of the piston and is usually chamfered/counter sunk, appearing bigger than it is but you'll need that small diameter wire to poke thru it.
    :lol3
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  17. lnewqban

    lnewqban Ninjetter

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    Besides all the above, it could simply be humidity in the brake fluid.
    Easy to happen if your mechanic used fluid that had been exposed to humid air (the brake fluid is highly hygroscopic).

    In some cases, that disolved small amount of water becomes steam with the heat from the pads; the additional pressure of that steam pushes the piston(s) forward until it locks the brake, then it condensates and unlocks the pads.
    Water expands by a factor of 1600 when it turns into steam at atmospheric pressure.
    :muutt
    #17
  18. Wentwest

    Wentwest How's that work?

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    Any of the solutions will mimic the symptoms you are reporting. I guess I'd try to clear out that pressure relief hole in the MC first, because it's the easiest and cheapest solution. Next might be to pump the MC with the bleeder open and a tube into a glass jar, and see if there's anything coming out in the fluid besides fluid. That would be a giveaway that the line is breaking down inside. You can look at the fluid after it stands for a bit to see if anything settles out.
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  19. mitchxout

    mitchxout Long timer Supporter

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    Thanks for the helpful info, I recently dealt with bad brake lines on my 04 r1150r. My rear caliper pistons wouldn't retract and bleeding revealed black bits in the fluid.
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  20. KennyFSU

    KennyFSU Look ma, no hands

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    Almost took a spill after a quick test ride. Removed the mc cap and nothing squirted up when squeezing the brake handle. I can’t see any debris in the bottom of the mc at all either.

    IMG_9408.JPG
    #20