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Counter-Seal

Front Brake Issue

14 posts in this topic

Hello all, 

 

So i've asked dealership/mechanics, old timers and friends about this issue and so far no one has any idea.

 

I was riding in Utah when i dropped my bike (sad yes i know but it happens) on the left side. The front brake worked great until that point, and when i found out it didn't have any pressure was when i was going down hill. I had to pump the lever to regain pressure. which they worked until i started moving again.

 

So heres the details on whats going on. When the bike is sitting still I have pressure, turn handle bar left or right it's good. I can leave it over night and come back and it still has pressure. The moment i hop on the bike(with and without the engine running) and put a load on the suspension i start to lose pressure. Within a distance of 10 ft i've lost 95 % of my pressure. If i pump the lever two times i'm back to normal pressure. 

 

Things i've done:

Ensured full reservoir and fluid change.   

Bled the system, on the bike, also took the whole system off the bike with the caliper raised above the master.

Checked for leaks in the system which i have none.

 

If you guy have any idea please let me know i'm mind blogged with this. 

Edited by Counter-Seal

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Any pulsing in the lever? Wonder if rotor is bent pushing caliper apart causing the loss of pressure...

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If your brake fluid level was low enough at the time you had it on its left side, the fill port at the bottom of the master cylinder could have risen above the surface of the fluid, allowing air to enter the brake line.  You may have a nasty little bubble living in your banjo fitting right at the lever.  Turn the bars so the the cylinder itself is tilted with the line end low, then loosen the brake line just a bit while you push the caliper piston back a little with a screwdriver between the rotor and pads. 

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Any pulsing in the lever? Wonder if rotor is bent pushing caliper apart causing the loss of pressure...

 

If he's described his symptoms right, that may well be the problem.

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Except for the fact that all he has to do is sit on the bike and he loses the pressure so the rotor won't be going around to push the piston back, if I understood his post correctly

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Except for the fact that all he has to do is sit on the bike and he loses the pressure so the rotor won't be going around to push the piston back, if I understood his post correctly

  It's not entirely clear but it sounds like it holds pressure until he hops on and rolls 10'.

Hello all, 

 

So heres the details on whats going on. When the bike is sitting still I have pressure, turn handle bar left or right it's good. I can leave it over night and come back and it still has pressure. The moment i hop on the bike(with and without the engine running) and put a load on the suspension i start to lose pressure. Within a distance of 10 ft i've lost 95 % of my pressure. If i pump the lever two times i'm back to normal pressure. 

my quick and easy check would be to lift the front wheel off the ground. get the pressure built up then turn the front wheel 1/8 of a turn at a time and re check the pressure at the lever. If you make it all the way around without any change, your rotor is probably good. If at one of the check points, you have to pump the lever 1-2 times to build pressure or you get An increased amount of drag at any time the rotor is bent enough to spread the pads each revolution.

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Well it looks like it's a bad rotor, it was bent, not much but made slight noise. ended up confirming that when i went to straighten it and made it worse, along with the problem. Looks like it was pushing the caliper back and making me "lose" pressure. i'll repost the update when i get a new one and test it....hopefully not on any downhill rides ;) Thank you all for your help, i'd still be stump for days.

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Well it looks like it's a bad rotor, it was bent, not much but made slight noise. ended up confirming that when i went to straighten it and made it worse, along with the problem. Looks like it was pushing the caliper back and making me "lose" pressure. i'll repost the update when i get a new one and test it....hopefully not on any downhill rides ;) Thank you all for your help, i'd still be stump for days.

Good deal. 10' was the giveaway. 10-12' is what it takes for the front wheel to Do one complete revolution.

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Ok I purchased a 2011 yz450f a couple of weeks ago. I rode it for about 15 minutes before I purchased it and then got it home and within ten minutes of riding the front brake locked up. I bled the system and that seemed to fix the problem. Then after a few more short rides around my house it locked up again. Caliper is hot and brake lever is tight. Bled it off again and it appears fine. What would be y'all's first thing to check on it that could explain this?

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Wrong brake lever , or a similar problem.  If levers for or from YZ's earlier than '08 are used, the master cylinder piston is pushed down the bore so far that it closes the fill/return port leading to the reservoir.  When that happens, fluid cannot move up into the reservoir as it heats and expands, and that traps pressure in the line that causes the brake to apply.

 

Even if you are sure that the levers are right, remove the lever at the pivot and watch to see how far the piston moves when the lever is removed or put back into place.

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It's all stock but since I've pulled it all apart and am checking everything I'll do as u say. One thing I'm thinking could've happened is the guy I bought the bike from had just replaced the fork seals. Wonder if something was in a bind when he re installed everything? Thank you for the help!

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