426 Master Cyclinder Issues

OK, here we go.....Round 4 on trying to solve this on going problem. '01 YZ426 front brake master cylinder causing fits. Bleed the unit once, worked for 1 ride, got mushy (meaning none existent). Bleed it again, same results. Took the bike to a shop, bleed it, same results. Took it to an "expert"....blah, blah, same results. (though this time it lasting a bit longer)

This is what I know (or don't)......after bleeding the brake it functions the way it needs to, after 1-2 rides it starts to show signs of degradation. There are no leaks! (meaning no signs of dirt accumulation around common leak points.)

What needs to be replaced? master cylinder, caliper, cable? Please help.

The only place in the system where air can get in without there necessarily being an external leak is the master cylinder. If you can bleed to the point that it is free of air and works, but more air is present later, air is getting in some place. If there are no external leaks it pretty much looks like the MC. Check for fluid under the dust boot that seals the plunger (where the lever bears on it).

It is possible that your master cylinder could be worn out inside, the rubber parts that is. This will cause a fluid bypass. To remedy this get yourself a master cylinder kit for about 14 bucks. It should come with about four pieces to rebuild your internals. I would also replace the rubber thingy (resevoir diaphram) under your mas cyl cap. Be sure not to overtighten the cap either, is it cracked? Be absolutely sure you have no leaks, I know you said you didn't. Sometimes fluid leaks can be teensty weensty (small). Lastly bleed using the method of pulling the caliper off the fork, put it into a vice CAREFULLY with the bleeder port sticking up at the highest point. Leaving it connected to your handle bars, begin bleed procedure. Again, the bleeder port need to be highest, even higher than your master cyclinder. :D If all this doesn't work, then trash the whole bike! :applause:

The mastercylinder kits for the Cr Honda's fit any of our YZ/YZF's and are about 1/2 price vs the yamaha part.


I am on my way to the shop to pick up the necessary parts. If this does not work you are all to blame! :D If it works then I am the hero, at least for a while. :applause:

Cheers. JB

Let us know...Glory Hog. :applause:


got buy a rebuild kit for the unit. Most likely have to torn a seal on the MC plunger. Should be about $17 retail for one. Then make sure you bleed out the MC first

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