Air in 640 Front Brake, Again!!

Discussion in 'Thumpers' started by clintnz, Oct 24, 2005.

  1. clintnz

    clintnz Trans-Global Chook Chaser

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    Went to pull the 640 ('03 Enduro) out of the shed this morning, (she's been there for 3 weeks all lonely while I've been holidaying in Europe with a newer model :evil) & discovered a very mushy front brake, When I had this a while back & I checked all the brake fittings & bled the system which fixed it, now it's happened again. Does anybody have any clues on where the air might be getting in? Do I need new seals somewhere?

    Cheers
    Clint
    #1
  2. creeper

    creeper Still alive...

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    You might want to completely flush the system with fresh dot 4 or 5.1 Clint... prolly has water in it.
    #2
  3. clintnz

    clintnz Trans-Global Chook Chaser

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    Did a full flush last time, the fluid was clean, but there was definitely air in there.

    Cheers
    Clint
    #3
  4. creeper

    creeper Still alive...

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    Strange... fluid is hydroscopic... but very little rubber in the system exposed to atmosphere, no rubber in the hose at all. Front side of the caliper piston O-rings... backside of the master cylinder O-ring… that's about it.

    Oh well... it only takes a little bit. :dunno
    #4
  5. yater

    yater Long timer

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    back bleed your system. I haven't seen anything on backbleeding on this site but it's BY FAR the easiest way to ensure there is no air in the line. Simply fill from the bottom nipple...takes 5 minutes and works everytime. If you don't find any info on parts needed when you search, pm me and I'll start a "how to" thread
    #5
  6. clintnz

    clintnz Trans-Global Chook Chaser

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    Cheers for that, but it's not the bleeding that is the problem, when I bled it last time the lever came up nice & hard, & it was good for the couple of months & several thousand km's until now. I'm really looking for any ideas on how the bike is getting air in the brake system while sitting in the shed??

    Cheers
    Clint
    #6
  7. creeper

    creeper Still alive...

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    It's obviously Air Elves.
    #7
  8. clintnz

    clintnz Trans-Global Chook Chaser

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    Of course... how could I be so silly...

    I'll remember to spray the bike liberally with Elvicide before the next time I go away.:D

    Cheers
    Clint
    #8
  9. creeper

    creeper Still alive...

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    There ya' go... :thumb

    You get South Park in NZ? There was a Underwear Elves bit a few years ago that was funny as hell.

    Ya' had to be there I guess. :dunno
    #9
  10. Donkey Hotey

    Donkey Hotey De Jo Momma

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    Here's my two guesses:

    1) Are you running glycol or silicone based fluid? I don't know if they use 'DOT' ratings for fluid down there but there seems to be confusion in the types of fluid and some people still insist on using silicone based fluids. There are many problems with silicone fluid which I'll spare you but the biggest problem is that it absorbs air bubbles that won't come out until the fluid is heat-cycled. I used to run the stuff many years ago but completely gave up. I was constantly bleeding the brakes after a change. It took 3-4 bleeds to get it all. Make sure you stay with a glycol based fluid and you shouldn't have this problem.

    2) It may not be 'air' but rather some other kind of gas being generated by active corrosion inside your caliper or master cylinder. As it corrodes, gas is given off and collects inside. Clean fluid won't allow that to happen but once it starts, it might be your source. In such a small caliper, a little gas goes a long way.
    #10
  11. Esteban

    Esteban Banned

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    master cylinder seal leakage
    #11
  12. Scoot Jockey

    Scoot Jockey BST Faithful

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    My 2003 Adventure does the same thing if I let it sit for more than two or three weeks. As far as a permanent fix, I haven't found one. Air must be creeping into the caliper somehow. Two tricks that have helped me are:

    1. First, I purge the caliper by carefully and firmly squeezing the piston side of the caliper towards the disc, thus creating a gap between the opposite "fixed" brake pad and the disc.

    2. Next turn your handlebars to the left, raising your fluid reservoir to the highest point. Hold the upper curve of the brake line down to create a downwards run from the reservoir to the caliper, then begin squeezing the brake lever slowly, and firmly to bring the air bubbles up. You could even have the cap off of the reservoir at this point to check the air release, and monitor the fluid level.

    A lot of times I will just do the second step, and it will make things rock solid again.

    Regards,
    Scoot
    #12
  13. Buckster

    Buckster Banned

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    I get this with my rear brake and plan on stripping the entire set up down for a look see, I suspect it is the seals in the piston chamber or something to do with the banjo's, if it is the banjo's I will replace the pipe with a custom Venhill unit, now winter is here I could really start needing the rear brake, otherwise I rarely use it.
    #13
  14. Taki

    Taki Waiting for Godot

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    I just had this problem with my hydrolic clutch this past weekend. I am very careful to purge all the air from the system periodically and completely flush the fluid twice a year. I noticed the clutch starting to loose its release power while on some technical trails, gradually the clutch started to fail over the next few hours. Each time I stopped and turned the bike off the clutch pressure returned only to dissappear again when the vibration started. Back at home I checked the fluid level and it was sufficiently high, but the entire line was full of "frothed" fluid. :huh My only guess is that air entered the system when the bike fell on its side and I grabbed the clutch, another possible reason for the brake line problems in this post.
    #14
  15. Trevor S

    Trevor S Cap'n Flatulence !

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    Undepants Gnomes :) Ranks in the top 5 episodes ...

    Business Plan
    1. Collect Underpants
    2. ...
    3. Profit.

    :lol3
    #15
  16. Hellcat

    Hellcat Le Grand

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    Same thing happens to me once in a while with my 02 adventure. My solution is to put the bike on the centerstand, sit on it so the front wheel is off the ground, turn the handlebars to the left and squeeze the front brake lever a few times till the pressure on the brake lever gets back to normal ( I also shake my right foot and rub my belly counter clockwise with my left hand while doing this....and when the pressure is back I get off the bike, do a little dance, then do a back flip with my right index finger up my left nostril then get on my stomac and do the worm dance while singing "we will rock you" from Quiet Riot...:rofl ). The trick here is to elevate the Master cylinder as high as possible so the the air can migrate into the master cylinder and not stay in the line.
    There is no leak of fluid anywhere on the system, the brake fluid level stays the same and Its a bit of a mistery to me where the air comes from. I've bled the system once before with complete brake fluid change etc...and it still the problem happens once in a while. If you guys find out where the problem comes from, please let me know. This problem is frustrating especially coming from a high quality manufacturer like Brembo.

    Cheers
    Hellcat


    #16
  17. clintnz

    clintnz Trans-Global Chook Chaser

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    Aaahh.. thats it! I had my left finger up my right nostril, It's bound to work next time...:lol3

    Thanks for the tips though, if I can get the air out without bleeding the system the problem is not so serious. I'll get the seal kits for the M/C & caliper anyway & swap them in next time I'm bored on a rainy day, I think Esteban is right about it being the M/C. Hopefully It's not a dodgy bore.

    I must have missed that episode of South Park, I'm not much of a regular TV watcher. I should get the DVD's sometime & catch up on the ones I've missed.

    Cheers
    Clint
    #17
  18. mikeyb

    mikeyb Just some guy

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    I had the same mushy brake after a few weeks of no use on my 625sxc.
    I searched ktmtalk and this seems to be a common problem with these brakes.
    The claim is that the design allows an air bubble to form where the banjo bolt goes into the master cylinder and that normal bleeding won't get rid of it.
    Some guys have drilled an extra hole into the banjo bolt so the air won't collect in the top half of the banjo. They claim that works well.

    I just went for the quick fix which is taking the master cyclinder off the bars and holding it vertically while pumping the lever to work the air up into the reservoir.
    I plan to try drilling the bolt the next time I change the fluid.
    #18
  19. Donkey Hotey

    Donkey Hotey De Jo Momma

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    I can give you a quick way of bleeding the brake without any tools (though you'll get some brake dust on your hands).

    Get down next to the caliper and disk. Reach your hands behind the caliper and put your fingers on the pads. Give a good steady squeeze. What you're trying to do is push the pads and pistons back into the caliper. If your pistons are clean and the caliper is working as designed, they should easily float right back into their bores.

    What this does is force the fluid back up to the master cylinder--with quite a bit of velocity if you do it right. Once you get that done, go back upstairs and pump the brake back up with the lever. Voila! Instant bled brakes.

    In truth, this is a little trick I've done for years to get all that trapped air out of the system. I vacuum bleed downhill to get it pretty close and force the remainder of the bubbles back up using the method described above.
    #19
  20. clintnz

    clintnz Trans-Global Chook Chaser

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    Thanks for the tips guys, it always seems to take a bit of jiggling before it bleeds up (although both times it has come good) so if next time forcing the pistons in don't do it I might try taking the M/C off & holding it up to see if there is air in the banjo.

    But how do I get rid of the Elves??? :lol3

    Cheers
    Clint
    #20