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Thread: What did I do to my front brake?

  1. #1
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    What did I do to my front brake?

    When changing my lever, some how I pulled the plunger and spring partially out of the rubber boot/grommet. I was having trouble getting it to go back in so I had this brilliant idea to pull it out completely and reinsert it. I was able to get it back into the small hole in the grommet by using a screwing action. Everything looked honky dory and I went for a test ride. A block from my house I noticed the brake lever was stiff with no pre-travel and then I came to a stop. The Master cylinder had pumped up and locked the front brake. All I could do was release some brake fluid from the bleeder screw to get under way again. Went home and after using the brake a couple of times it pumped up and locked again. I bled again and it pumped up again. I suspect the spring that is attached to the plunger has something to do with my problem.

    Any suggestions? Looks like I can't ride until this is fixed.

  2. #2
    apriliaforum expert Tifa's Avatar
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    You have not got enough free play in the plunger.
    This causes the brakes to drag.
    Which in turn generates heat, causing the fluid to expand, increases pressure in the system, locking up the front.

    Re-adjust the plunger and it will be fine.
    If you've damaged it, replacements are available from AF1

    You were lucky, a few with this problem have been spat off their bikes at speed when the wheel locked up.

    You didn't by any chance fit some cheap aftermarket chinese/ebay levers ?

  3. #3
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    How do you adjust the plunger? The odd thing is, there is adequate free play to begin with and after pulling the lever a few times, even without moving, the free play goes away.

    Chinese levers? Yes but, I had them on for a few weeks and they worked well. I broke one moving the bike in close quarters, put the original back on, then put another Chinese one on. Thats when my trouble started. I'll put the original back on and see if that helps.

    Quote Originally Posted by Tifa View Post
    You have not got enough free play in the plunger.
    This causes the brakes to drag.
    Which in turn generates heat, causing the fluid to expand, increases pressure in the system, locking up the front.

    Re-adjust the plunger and it will be fine.
    If you've damaged it, replacements are available from AF1

    You were lucky, a few with this problem have been spat off their bikes at speed when the wheel locked up.

    You didn't by any chance fit some cheap aftermarket chinese/ebay levers ?

  4. #4
    apriliaforum expert Tifa's Avatar
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    Quote Originally Posted by aimhigh View Post
    How do you adjust the plunger?
    the plunger screws into a barrel at 90deg

    if you look carefully at one end of the barrel, there is a very small (tiny) allen grub screw.

    both the allen svrew and the plunger are likely yo be tight because they are usually loctited in.

  5. #5
    apriliaforum expert kzmille's Avatar
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    I don't think the plunger is threaded on the brake master. At least it is not on my 01 Mille. The Tuono's master has a different part number but I would expect them to be the same in this regard. This is a fairly common event when fitting after market levers. The problem is as Tifa suggests. There is no free play between the piston and plunger. With the new lever the plunger is effectively too long. This pushes the piston in too far before you even pull the lever. This blocks the supply port in the master preventing fluid flow between the reservoir and the cylinder. The only fix is to shorten the plunger just enough so the port is open.

    You can cut the shoulder back if you have a lathe or grind the rounded end.
    Attached Images Attached Images   

  6. #6
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    kzmille, your pictures look like my parts- no threading like on the clutch. I re-installed my original lever for now. Instead of modding the plunger, what about modding the after market lever? Are replacement plungers available? I'd feel better about modding the plunger if i knew I could replace it. BTW, what is the function of the spring on the plunger?

    Thanks kzmille.

  7. #7
    apriliaforum expert Tifa's Avatar
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    Quote Originally Posted by kzmille View Post
    I don't think the plunger is threaded on the brake master.
    Yes, my mistake, you are correct, it's the clutch that has the threaded barrel.

  8. #8
    apriliaforum expert kzmille's Avatar
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    The spring on the plunger is just to take up what ever free play there is and keep the lever against its outermost position.

  9. #9
    apriliaforum expert kzmille's Avatar
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    I don't know off the top of my head if the lever itself could be modified to end the problem. It looks like you could file the stop on the lever pivot part. That might not be acceptable either if that is your original.

    Odd that they chose such different designs between the brake and clutch. It seems they went with no adjustment on the brake to make it fool proof but it wasn't enough to prevent problems with rather poorly executed knockoff levers.

  10. #10
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    I was looking at the parts diagram on AF1's store site http://www.af1racing.com/store/Scrip...idCategory=236 and the plunger doesn't have a part number. Does that mean its not available separately? I'd rather not mod the original part if I can't replace it. I could make one but, thats a PIA for such a small part.

    Quote Originally Posted by kzmille View Post
    It seems they went with no adjustment on the brake to make it fool proof but it wasn't enough to prevent problems with rather poorly executed knockoff levers.
    Nothing is fool proof because fools are so ingenious!

  11. #11
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    Quote Originally Posted by kzmille View Post
    I don't know off the top of my head if the lever itself could be modified to end the problem. It looks like you could file the stop on the lever pivot part.
    Thats what I was thinking but, I realized that would result in the lever being further from the bar. I want the lever closer to the bar. I like having my cake and eating it, too!

  12. #12
    apriliaforum expert cokoop's Avatar
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    I think I did the same thing. Is this the plunger we are talking about? It looks like it comes with the brake lever, part no. BR110950056

    http://www.af1racing.com/store/Scrip...idproduct=4279

  13. #13
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    That is the plunger and the bushing together. I sure don't want to buy the whole lever just to get the plunger. I'd make one before I'd do that.

  14. #14
    apriliaforum expert kzmille's Avatar
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    Quote Originally Posted by aimhigh View Post
    Nothing is fool proof because fools are so ingenious!
    That is a great quote, by the way.

  15. #15
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    Quote Originally Posted by kzmille View Post
    That is a great quote, by the way.
    I wish i could take credit for it. I just prove it's validity now and again.

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