Bad Master Cylinder : (

2 replies to this topic
  • 642MX

Posted July 29, 2007 - 03:19 PM


The wifey and I went riding today in the scorching heat. She pulled up next to me and said her front brake wasn't working. We made our way back to the truck and suddenly it started working again. I went ahead and flushed the fluid anyways.

Long story short, the front brake faded again, but would sometimes work. I guess the master cylinder went bad. I've never seen one intermittently fail/work.

I don't believe it has air in it. I think it must be junk. Any ideas?

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  • grayracer513

Posted July 29, 2007 - 03:58 PM


The forward seal on the piston is probably getting worn, shrunken, or stiff, or loosing its resilience. They seal in one direction only by design (when pushed down the bore), and the pressure pushing against the lips of the seal and forcing them into place is what makes them work. When they get too old, they sometimes will start to blow fluid by the seal instead of flaring open under pressure and sealing as they should. If the bore's good, a kit should fix it.

  • 642MX

Posted July 30, 2007 - 04:31 AM


Thanks gray. I see where they sell a kit, but since its my wifes bike, it will get a whole new master cylinder. :ride:

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