Leaking Rear Brake Cylinder

5 replies to this topic
  • tmasWR

Posted July 23, 2012 - 02:12 PM


Well I took my first little spill on my WR450 yesterday on some bouldery switchbacks. It wasn't much of a wreck, but it did mess up my heat shroud on my header, and bent my brake lever up under the crank cover. I was able to bend it back out to where it looks like it never happened, jumped back on the bike and brakes worked, and headed down.
By the time I got home my master cylinder was dried up and no brakes. SO, being new to dirt bikes, i'm not sure what the diagnosis is.
I can tell the fluid is leaking from the bottom, i'm guessing the exposed rubber is just a dust boot not an actual fluid seal, i'd guess the actual fluid seal would be inside the housing, but it is coming from around this exposed boot..

So is the leak caused by a damaged seal, or is my lever still bent just slightly enough that it's causing mis-alignment of the "ram" or rod that goes up into the master cylinder?
I refilled with DOT4 fluid, bled out the rear brake, and sure enough it still leaks when brake is applied.

What do you guys think i should do to diagnose or repair this. Rebuild Kit for $25 online? Or do you think it's gonna be my lever. Is there a way to tell? Thanks a lot, great information on this board.

  • MANIAC998

Posted July 23, 2012 - 05:13 PM


It's possible that the lever is mis-aligned, but un-likely. My guess would be that the seal is damaged, and replacing it will fix the problem. Why don't you pull off the dust boot, and take a look at the plunger alignment?

  • Billy450

Posted July 24, 2012 - 03:04 AM


Could have bent the rod inside the seal. I would take the boot off and check, if need be strip down the cylinder and check all the seals.

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  • Bandit9

Posted July 24, 2012 - 07:53 PM


I think the manual is pretty good at describing the servicing/rebuild of the master cylinders. I did it once in 05, if I can do it, anyone can. Ace wrench I am not, but i manage.

  • tmasWR

Posted July 25, 2012 - 02:22 PM


Thanks guys always great help here. I should learn how to rebuild this cylinder for sure, i have an electronic version of the manual i'll have to look through it and see what i can find.
It turned out to be a really easy fix. It still leaked fluid after refilling and bleeding twice, I just used my calipers and measured the alignment on the rod going up into the cylinder, it was off by a few thousandths, i slightly bent the back side of the pedal until it was lined up really close, and then I pumped the pedal several dozen times. Filled again, bled, and it's been good for 2 days driving to work with no leaking! I hope it holds up, if not i'm guessing based on what I did it would still be slightly out of line, or a seal inside would be damaged from the bend. Good to go for now! Ride on....

  • MANIAC998

Posted July 26, 2012 - 03:46 AM


Sweet! You got lucky!!


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