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Thread: Rear brake issues... piston seized? Or not enough pressure?

  1. #11
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by AC_Schnitzel View Post
    St Paul HD doesn't have a rebuild kit for the rear caliper... Anybody know an equivalent part number for the Nissin assembly?
    You asked for the part number for the assembly, not a rebuild kit.

  2. #12
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by AC_Schnitzel View Post
    My neighbor has a compressor -- might ask him if I can use it to push the piston out.

    I'm also going to rebuild my rear MC first and see if that fixes the issue.
    Wait? So the master cylinder isn’t working? Why are you messing with the caliper if the master cylinder isn’t working? Or are you just guessi.... ohhhh never mind.... i get it now. Maybe try a TPS reset while you’re at it?

  3. #13
    Member AC_Schnitzel's Avatar
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    My question still remains. How do I discern between failing MC vs seized/corroded caliper piston.

    Symptoms:
    - brake lever doesn't build enough pressure to push piston out (though it can push fluid through the bleeder)
    - caliper piston was difficult to retract (reservoir was off)

    I took the entire assembly off the bike and will rebuild both since the parts are relatively cheap.

    Quote Originally Posted by 34nineteen View Post
    You asked for the part number for the assembly, not a rebuild kit.
    My mistake, I meant the rebuild kit for that model caliper (not available w/ them).

    EDIT: I will try to bleed one more time -- maybe there is air stuck at the top of the brake line near the MC.
    Last edited by AC_Schnitzel; 04-06-2020 at 01:14 AM.

  4. #14
    Senior Member 34nineteen's Avatar
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    Take m/c, hose, and caliper all off the bike so you can hang them to float the air pocket out if needed.


    Is the pedal stopping because of a “hydraulic lock” or is the piston in the m/c at it’s mechanical end point?

    If it locked “hydraulically” then you should be able to push the piston out with that force. Unless it’s been sitting in seawater, it will break free under your weight. Also, make sure your piston actually has room to move to free itself. You may need to remove it from the rotor, remove the pads and shove a shop rag in there to cushion the blow when it breaks free. You may want to wrap it in a old towel or something to catch the brake fluid from flying anywhere.

    You can also loosen the hose at the top of the m/c to see if the m/c is actually pushing fluid.

    If it’s just reaching the end of the throw, you have an air pocket, which is cushioning the hydraulic force from moving the piston.

    I used 1oz plastic syringes for my bleeds. I prefer to push the fluid back upward towards the reservoir.

  5. #15
    Member AC_Schnitzel's Avatar
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    the MC piston is at its end point. Thanks for the tips. I will try those things tomorrow, starting with bleeding at the banjo bolt at the MC.

  6. #16
    Senior Member 34nineteen's Avatar
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    I’d recommend taking off the heel guard and tying a shop rag around it to catch spills.

    Good luck !

    Ps. I actually use a 2 oz (60mL) syringe for my bleeds. YMMV, but that’s what works for me.


    Last edited by 34nineteen; 04-06-2020 at 04:32 AM.

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