Failing brakes!!

My rear brake will just fail on me out of the blue. I'll apply it again and it engages.

Its so random its scaring me. I've almost rear ended a couple of bikes and one time just flew over a berm.

What is causing this. Is it air in the line?


If it was air in the line, it would be the same all the time. The seals in your master cylinder are leaking, this may not be a visible external leak but inside the brake fluid is passing the main seal back into the reservoir, but check externally to make sure this is the case...You may be able to replace the seals but if the master cylinder is scored or pitted replacement is the only option....Best Of Luck.....CURTAINS23

Sounds like the master cyclindar to me aswell. Hopefully its still in good enough condition to rebuild it.

It sounds like a MC to me too, as he said. If you aren't able to fix yours, and it's an '02 or earlier with the separate fluid reservoir, let me know. I have an extra.

Is your rotor bent :naughty:

Is your rotor bent ?
Now, that's a good question. :naughty:

If it is, your symptoms should be that the more frequently you use the brake, the more likely it is to work, and the longer you go between uses, the more likely it is to act up. A wobbling rotor will push the piston back into the caliper a bit at a time and increase the pedal travel. It wouldn't really be random; there would be a pattern based on how far you went without using the brake. Anyway, a bent rotor is easy to see.

Rotor looks straight. Its pretty random, after the first time it happended I pulled of and in the pits just rode slow and kept engaging it, not a hint of failure.. I'll look at the master cylinder.

I really know nothing about brakes, I've just changed the fluid. Any tips on inspecting the mc would be greatly appreciated...

I've got an 01. Thanks for the replies, a bad mc would've never came to mind.

When you fill the system, be sure to use DOT 4 only, like it says right on the parts. DOT 3 and DOT 4 are not entirely interchangeable.

On the same topic.. I purged my rear brake when i lowered the pedal. (Motul Dot 4)

After i was done, the brakes went from good to worse. I've bled dozens of brakes and have always been good at making them better simply by bleeding/purging. I know theres NO air in the line but they're not grabbing anywhere near as hard as before. I did the front at the same time with a Very noticable improvment. The part that has me going is that how did I go from OK, to weak on the rear by just pumping fresh fluid through? I'm going on 8 ounces of careful bleeding and it aint gettin any better. Any Suggestions?

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