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rear brake

Discussion in 'Motorcycle Tech Talk' started by joshbob, Sep 18, 2014.

  1. joshbob

    joshbob Well-Known Member

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    Tattoo and I cannot get the rear brake on a Sportster bobber he built for a customer to work. The componants are a Tokico caliper and a metric sport bike rear master cyclinder (Kawasaki, Triumph, ect.). Both componants are brand new. We spent hours trying to bleed the system. Even with the air apparently bled out, the master cylinder still won't hold. We've tried bleeding with the master cylinder in a vertical position, horizontal position, with the caliper right side up, upside down, on and off the rotor (with a flat piece of steel between the pads), with the master cylinder higher than the rear caliper, every which way but loose.
    This is the second Master cylinder he's tried. The first one was used and he rebuilt it. That one didn't work and so he ordered a new one and the same problem persists.
    We took the new master cylinder apart yesterday and inspected the parts and everything looks good. WTH?

    He is using DOT4 fluid. Maybe we should try DOT5? Also the brake line he is using is that small diameter black plastic looking stuff. I suggested changing to a larger braided line and/or to DOT5 but he doesn't think that those changes will help. Any suggestions would be greatly appreciated.

    Something else I noticed: when the master is pumped up, the brake fluid level in the reservior drops slightly and when you release the plunger it leaks back up into the reservior. Is that normal?
    Last edited: Sep 18, 2014
  2. Lucifer

    Lucifer Well-Known Member

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    Are the caliper pistons moving freely??Don't switch from DOT4 to 5,if you're using 4 stick with that...
    I put a new caliper on the Heritage years ago and had to use the compressor to blow air in the caliper to get the pistons moving,couldn't get enough pressure from the brake fluid to free them up...I put the pads in and a putty knife between the pads,just make sure you don't pop the pistons from their bores...
  3. baggerpaul

    baggerpaul Well-Known Member

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    Bleeder on the caliper must always face up or you will never get the air out . Try this gravity bleed the system . Then allow the piston in the caliper to move a bit while you pump it 2or 3 times then with the lid off the master and in place on the bike force the piston back in the caliper .check for air in the master as you do it repeat as needed
  4. Roadster guy

    Roadster guy Well-Known Member

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    Are you pumping the lever slowly by hand several times before opening the bleeder? If you pump too rapidly you can aerate the fluid. How about a hand vacuum pump to suck the fluid from the master through the system? Maybe the master cylinder used has too small capacity for the bigger Japanese caliper. Are they from the same donor bike? Just throwing out ideas.
  5. hotroadking

    hotroadking Super Moderator Staff Member

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    it's probably in the line, those rear brakes can be a PIA tap along the line with a small screwdriver handle all the way, might be a jammed in air bubble thats letting some fluid by.

    I use one of these

    http://www.mityvac.com/pages/products_bcbe.asp

    [​IMG]
  6. baggerpaul

    baggerpaul Well-Known Member

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    Yep that will work also .i find that if i can get the piston to move out about a half stroke as i push the piston back i unlock the air bubble . If you continue to pump the master you just air up all the fluid .having the master lower than the caliper is a pain in the arse to get the air out.
  7. bambidee

    bambidee Active Member

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    as BP said the bleeder has to be at the highest position in the caliper for it to bleed out air. Even when the system is working correctly when you release the master a small amount of fluid will return, but a larger amount would indicate sticking hardware creating excessive piston travel or air in system. Have also had luck pushing piston out to full travel and then pushing piston back to push air out of system as a backwards bleed, can be messy if you push piston out to far or are not ready for the amount of fluid that will overfill the master.
  8. FLHTbiker

    FLHTbiker Moderator Staff Member

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    That's how I do mine also works great
  9. joshbob

    joshbob Well-Known Member

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    Tattoo don't have one of those bleeder tools. Maybe he should invest in one. He has never had a problem with standard brake setups, but this one is "custom" and giving him the blues. When I went back to see him with the info I'd gotten from you guys, he had already packed the whole system up and sent it to a guy across town. He claims he already has over a whole day in lost labor and was fed up messing with it. When mounted on the bike, the caliper is about the same height as the master cylinder.
    Yup, at times the brake was pumped up a bit fast. Apparently, the caliper pads were working as they would grab the rotor when pumping up the master, and held when we tried pushing the bike, but when the brake was released and applied again, it would not hold. I'm thinking trapped air somewhere. But how can you have trapped air after hours of bleeding and rebleeding?
    I sometimes did notice (once) some very, very fine air bubbles, after we had gotten all the large bubbles cleared out. At other times, though, the fluid coming out of the system when bleeding was perfectly clear.
    Last edited: Sep 19, 2014
  10. Roadster guy

    Roadster guy Well-Known Member

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    Vacuum hand pump kit can be had for around $50.00. Get your buddy an early xmas gift.
  11. hotroadking

    hotroadking Super Moderator Staff Member

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    Halblor Flreeght has them $39 online
    http://www.harborfreight.com/mityvac-vacuum-pump-39522.html

    there are little turns and twists with that hard line and it
    can trap a lot of little air bubbles.

    IF you were getting it pumped up and it's loosing pressure then
    it's very possible there is a leak internally
  12. joshbob

    joshbob Well-Known Member

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    Haven't got the stuff back yet. It'll be interesting to learn what the problem was.
  13. joshbob

    joshbob Well-Known Member

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    I think I will.
  14. Lucifer

    Lucifer Well-Known Member

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  15. joshbob

    joshbob Well-Known Member

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    I suppose it's possible that the master is too small for the caliper. The bore in the master is maybe 1/2" or so. But I've seen similar setups on various metric bikes and they worked fine. Who knows . . .

    This can happen when you mismatch componants not intended for each other.

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