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Another front brake question

Discussion in 'The Garage' started by bosco659, Aug 8, 2020.

  1. bosco659

    bosco659 XS650 Addict XS650.com Supporter

    On my ‘76 with an R6 dual disc setup, while riding and applying the front brake, I experience random excessive travel of the brake lever. To try to explain this better, when the lever is normally squeezed, the lever is say 3/4” away from the throttle grip and firm. Then sometimes (not often), when I apply the brake, the lever will travel further than “normal” to approximately 1/4” from the throttle grip. When this happens, if I release the lever and squeeze it again, I get the 3/4” spacing.

    I have bled the master cylinder and both calipers and there are no signs anywhere if a fluid leak. The master cylinder has a separate reservoir and the master cylinder itself looks like an eBay special. I will most likely replace the complete unit very soon but this is puzzling in the absence of a fluid leak.

    Any ideas. I can post a few pics tomorrow. Thx
    gggGary likes this.
  2. bosco659

    bosco659 XS650 Addict XS650.com Supporter

  3. Brassneck

    Brassneck XS650 Guru

    Does this "longer pull" occur after a lot of riding, or is it random when it happens? I had an old '79 FJ40 Landcruiser back in the day...did the same thing...normal pedal most of the time, then every once in a while it would drop all the way to the floor....releasing it, brought it back to "normal"... Seemed to do it most when it got hot.

    Ultimately, I couldn't ever figure it out on my own...I bled that thing so many times, thinking I had air or water in the lines. An old-timer at the shop I worked at suspected there could be a bad seal on the MC that would work most of the times, but not all the time. My solution was to get a new MC...that fixed it. I'm sure there was a way to cure it, but I was young and didn't have the interest or patience.
  4. Steve C

    Steve C XS650 Addict

    If random, could be a bad master cylinder. If it repeats or happens when brakes get hot, possibly a warped/warping with heat rotor. Had a ZRX with a warped rotor do this but it was every time you’d get rolling. The rotor would throw the pads back further than normal and you’d have to double pump to get a proper feel on the brakes. Pretty scary feeling.
    Brassneck likes this.
  5. bosco659

    bosco659 XS650 Addict XS650.com Supporter

    It happens randomly, not necessarily when hot. I get that it is most likely bad seals in the master cylinder, but if the fluid is bypassing the seals, shouldn’t there be a fluid leak? It never gets so bad that I lose braking capability altogether. It just surprises you when the lever comes in that much more with no forewarning.
  6. Brassneck

    Brassneck XS650 Guru

    My guess is it goes back into the reservoir?
  7. Steve C

    Steve C XS650 Addict

    A faulty M cylinder can bypass internally w/o leaking fluid
  8. DaveO

    DaveO XS650 Addict

    Looking at diagrams of a master cylinders it seems that they have two ports a vent port and a replenishing port. When the seal passes over the second of these, both are inaccessible to the fluid in front of the piston and pressure builds up.

    If fluid were to suddenly leak past the seal it would just go up one on these ports and into the reservoir. So no leak.

    I would guess that a seal which catches up on the wall of the master cylinder would give you this problem. So either a poor seal or an imperfecton, scratch or muck on the cylinder wall is what I would be looking for.

    (As it's brakes, LOL, This advice is given "without prejudice" :) )

    lakeview likes this.
  9. bosco659

    bosco659 XS650 Addict XS650.com Supporter

    Yes maybe it could bypass back into the reservoir. Time for a new one. Thx

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