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Thread: Rear Brake analysis, what is going wrong?

  1. #1
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    27th January 2014 - 15:10
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    Rear Brake analysis, what is going wrong?

    So i have just got a new bike, has been sitting round for a few years.
    The rear brake is super spongy so I came to the conclusion that it must be old fluid, water air in the line.
    Have just bled the rear brake with new fluid.
    The problem is still there.
    you push the brake pedal once and there is nothing the but if you repeatedly pump them you get very minor braking,
    I am unsure where to go from here, brake lines or seals?
    I know it is losing pressure somewhere but it is not losing fluid.....
    any tips on if it is more likely master or slave?
    Cheers
    Nobbsie

  2. #2
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    9th May 2008 - 21:23
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    Visible leaks?

    In the absence of a visible leak, if you're losing pressure with the pedal depressed I'd look at the master cylinder, may be as simple as replacing perished seals.

  3. #3
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    No visible leakage at all!
    That was what i was thinking to might try pull it down tonight and have a look
    Cheers

  4. #4
    Seized slide pins.
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  5. #5
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    Old bike I like to pump out the pistons (pads out slowly pump both pistons evenly till they can be wriggled out by hand top up if required) pull out the seals clean the white powder behind the seals and inspect seals.

    If in doubt new but main seals that are square usually last. Check sliding pins if fitted. Clean bore inspect for corrosion. Light scotchbrite or replacement same with pistons.

    That should bleed up well after a long time expelling air.

    If not then master cylinder but your time has not been wasted.

    Some oddballs have to be upended to get the bleed nipple to the highest point but unlikely on a jap bike.
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  6. #6
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    As above pull the caliper off and check/clean. Caliper kits are what, $20?

    Also check the line hasn't gone soft and is 'ballooning' out at some point when pressure is applied, but do the caliper first.

  7. #7
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    Good as gold i might do that too
    thanks so much for the info guys

  8. #8
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    Quote Originally Posted by Tazz View Post
    As above pull the caliper off and check/clean. Caliper kits are what, $20?

    .
    shit, i missed the bit where they said make and model. what about alignment, does the whole caliper move when the brakes are applied, part of your travel could be taken up with allowing for this movement. take the caliper off and appy it directly to the disc and see whether it works better.

  9. #9
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    No caliper does not move,
    Sorry it is a gsxr 400 1991

  10. #10
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    Quote Originally Posted by Nobbsie View Post
    No caliper does not move,
    Sorry it is a gsxr 400 1991
    Do the pads move inside the calipers?

  11. #11
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    fuck me a helpful thread on KB.

    you're lucky noo guy. fokken lucky.l

  12. #12
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    Quote Originally Posted by Nobbsie View Post
    The rear brake is super spongy so I came to the conclusion that it must be old fluid, water air in the line.
    Have just bled the rear brake with new fluid.
    The problem is still there.
    you push the brake pedal once and there is nothing the but if you repeatedly pump them you get very minor braking,
    I am unsure where to go from here, brake lines or seals?
    I know it is losing pressure somewhere but it is not losing fluid.....
    any tips on if it is more likely master or slave?
    Cheers
    Nobbsie
    Sounds like air is getting in ... probably through the bleed nipple. (but not necessilary)

    Check also the resovoir lid seal. (leaks often show below fluid level)
    When life throws you a curve ... Lean into it ...

  13. #13
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    Pistons will be stuck to the caliper seals & will be pulling the pistons back further than normal when the brake is released, requiring more fluid displacement to operate the brake.
    Strip the calipers, clean out the seal grooves with a scriber wire buff the pistons & wash the whole lot, seals, dust boots calipers & all in warm soapy water. Reassemble with red rubber grease on seals & pistons, bleed up & all will be well.

  14. #14
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    Quote Originally Posted by sidecar bob View Post
    Pistons will be stuck to the caliper seals & will be pulling the pistons back further than normal when the brake is released, requiring more fluid displacement to operate the brake.
    Strip the calipers, clean out the seal grooves with a scriber wire buff the pistons & wash the whole lot, seals, dust boots calipers & all in warm soapy water. Reassemble with red rubber grease on seals & pistons, bleed up & all will be well.
    This guy. 1+
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  15. #15
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    Quote Originally Posted by jellywrestler View Post
    shit, i missed the bit where they said make and model.
    Fair call, but a bunch of cars and 3 bike brands deep and I'm yet to come across exorbitant prices for some seals + whatever else it needs in the way of circlips and rubbers.

    http://www.ebay.ie/itm/GSX-R-400-RL-...item1e9538e6fa

    Looks like you can get a new piston and seals for $50< for this model, but the advice above should suffice.

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