Rear Break Center Piston does not feed back in after braking

Discussion in 'Mechanics Garage' started by JWT87, Feb 4, 2018.

  1. JWT87

    JWT87 Member

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    After much troubleshooting: Replacing the brake fluid, disassembling the rear caliper completely and cleaning, pressure washing components, I have run out of ideas on this brake problem.

    the center piston on the rear brake extends just fine with front left brake rotation while the brake is engaged.
    the problem arises when the brake is let off, the center piston does not retract. even with the caliper removed and a clamp applied the brake does not return to the housing.

    *side note* when the brake bleed nipple is loosened the piston retracts into the housing easily.

    I am not sure how to proceed, any ideas?
     
  2. squirrelman

    squirrelman Well-Known Member

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    Fix it before riding !

    maybe try flushing alot more brake fluid through and re-bleeding ?

    didn't say ABS or linked or older type ? check for over-filling of fluid, dirt in mc reservoirs.
     
    Last edited: Feb 4, 2018
  3. NorcalBoy

    NorcalBoy Well-Known Member

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  4. squirrelman

    squirrelman Well-Known Member

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    EXACTLY, ya gotta read the BOOK first :rolleyes: then ask about anything you don't understand.
     
  5. Terry Smith

    Terry Smith Well-Known Member

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    I think you may find the fault is within the secondary master cylinder on the fork leg. Like a conventional master cylinder, these can get a small bit of crud in the tiny return port and so are unable to release fluid pressure when you get off the brake. A good flushing may move this on.
     
  6. OZ VFR

    OZ VFR Well-Known Member

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    ^ As Terry said.
    You can remove the secondary master cylinder and open it up, also remove the lines and check for moisture sludge and corrosion.
    Also check that the return spring is OK.
    The best flushing agent is actually hot water. Just make sure it's dry before reinstalling.
     
  7. vegaquark

    vegaquark Active Member

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    This happend to me with the clutch and it was as simple as the relief tiny hole in the clutch pump being clogged... It was more noticeable when the fluid heated up... Be careful! This is a major issue!...
    In your case being the front left hand caliper the pump, maybe the relief hole is there... Or your proportional valve is clogged as they suggested.


    Sent from my iPhone using Tapatalk
     
  8. JWT87

    JWT87 Member

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    Thanks for the Idea! after removing the secondary master cylinder and the Proportional control valve and cleaning everything out i am still having the same issue.
    are there any other areas where the returning fluid can get stuck?
     
  9. OZ VFR

    OZ VFR Well-Known Member

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    You need to start looking at where the blockage is.
    Apply brakes so that pressure is built up on rear brake center piston.
    Then start loosening bleed nipples one at a time starting with the secondary master cylinder.
    If pressure is relieved there, blockage is at the SMC, if not start going back all the way to the rear caliper.
    Loosen one line or nipple at a time until you see the pressure being released.
     
  10. squirrelman

    squirrelman Well-Known Member

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    Brilliant, that is ! ;) 'old me kangaroo down mate !
     
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