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Old 12-28-2016, 03:27 PM
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Default YZ125 Front Brake lockup

     

We have a 2016 YZ125 with less than 12 hours on it and our son was riding it today and pulled off the track because he thought the tire(s) were flat because the bike wouldn't go. When we looked closer, the front wheel wouldn't turn at all, nothing jammed, brakes not pulled. I started to take off the front wheel and then like magic, the wheel turned. I put it back together and it was fine. He ran about 2 more laps and it did it again, brake disc blazing hot and front wheel not spinning. I thought to bleed the front brakes so I did. Seemed to work, wheel started turning again so he returned to the track. Again, about 1 1/2 laps in, he takes a jump, front wheel locked up and he endoed (thankfully didn't crash) but he was done. Anyone know what may be happening? Do I need to do a more thorough brake bleed or do you think there is something more going on? Thanks in advance for any help or tips or suggestions.
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Old 12-28-2016, 11:14 PM
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sounds like the caliper pistons are sticking or the brake pads maybe not installed correctly?

probably it is time for a full caliper check or rebuild, check for

1) proper brake pads installation
2) clean/inspect/grease (very lightly) brake pad pin
3) clean/inspect pads retainers
4) clean/inspect/lubricate caliper pistons, if they are stuck and do not operate properly you could use soapy warm/hot water to clean them initially, by using the brake lever pump them out (not too much, about 1/2inch) re clean, then dry them out and then use silicone spray to lube them. Then try to work them in/out to allow the lubricant to penetrate, you'll notice that they will work smoothly onto their bore.

5) clean/inspect/lubricate the caliper pins, use waterproof grease

having said that you'll probably need new brake pads as these are now fried many times.
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Old 12-29-2016, 06:58 AM
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If you have too much fluid in the master cylinder it will not allow the piston to fully return and cause the breaks to drag and lock up when the fluid expands from the heat.

Paw Paw
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Old 12-29-2016, 10:14 AM
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Thank you both, I will work on the caliper this weekend. As for the master cylinder having too much fluid, we haven't added any, would they have filled it too high at Yamaha? Would it have taken 12 hours for this to occur? Any more tips or advice appreciated, my son is now a little nervous about riding as the double he endoed scared the crap out of him. Thanks again and Happy New Year!
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Old 12-29-2016, 09:32 PM
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like said rebuild caliper and greASE the pistons and look for condition of seals , pads and pistons and replace parts if needed. then bleed caliper. get back on horse the 1st wreck is the scariest. good luck
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Old 12-30-2016, 03:55 PM
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Quote:
Originally Posted by shaneman View Post
Thank you both, I will work on the caliper this weekend. As for the master cylinder having too much fluid, we haven't added any, would they have filled it too high at Yamaha? Would it have taken 12 hours for this to occur? Any more tips or advice appreciated, my son is now a little nervous about riding as the double he endoed scared the crap out of him. Thanks again and Happy New Year!
Sometimes dealers aren't the best at working on the products they sell. My local dealer did not know that shock reservoirs had nitrogen... I would start with the free and easy thing first: check the brake fluid level. If that is fine, then I would move on to assessing the calipers and rotor.
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Old 12-30-2016, 09:04 PM
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Ok, I took apart the caliper, removed the pistons and cleaned them. One of the pistons was a little stuck so that may have been a cause of the brakes ceasing up. I have put everything back together, now the pistons won't move at all when I pull the brake and the brake lever feels like it has no resistance. I've bled the line over-and-over by pulling on the brake lever and letting air leak out of the bleeder about 30 times and still nothing. I made sure there was enough DOT4 in the brake fluid reservoir. Am I not being patient enough bleeding or am I doing something totally wrong?
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Old 12-31-2016, 02:02 AM
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You may still have air in the lines. The master only move a very, very small amount of fluid and thus has a hard time of pushing the air out. I have always used reverse bleeding on these brakes. You can do a search on you tube to find out how to do this, but the basic idea is to have the caliber elevated above the master and flow the fluid from the caliber to the master.
It could also be that the seals are bad in the caliber. If dirt got in there then so can air.

Paw Paw
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Old 12-31-2016, 09:56 AM
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blow the pistons back out with air, grease the pistons lightly with high temp
grease assemble caliper. also check condition of pistons like for rust, scratches .
good luck
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Old 12-31-2016, 01:09 PM
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I've attempted the reverse bleed but it will not let me push the brake fluid into the bleeder when I open it. The fluid comes out when I was traditionally bleeding it so I know there's not an issue going out, why would I have an issue reverse bleeding?
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Old 12-31-2016, 05:43 PM
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I had the same issue when I put new pads on our 2012. Tried everything. I finally noticed the issue was occurring when I was tightening the caliper and spinning the wheel. It would drag at this point. In the end I had to use a thin washer on one side for it to spin freely. We bought the bike used 1 year old and looked perfect. Not sure if it had been tweaked prior but this was the fix that has continued to work for me.
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Old 01-02-2017, 06:37 AM
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I have seen it on older cars when the flexible brake line collapses and the fluid can't return to the reservoir. I can't imagine it would occur with only 12 hours, but it is something you can easily check.
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Old 01-10-2017, 12:44 PM
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Quote:
Originally Posted by Brett Campbell View Post
I have seen it on older cars when the flexible brake line collapses and the fluid can't return to the reservoir. I can't imagine it would occur with only 12 hours, but it is something you can easily check.
That might be the root to the entire issue.
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Old 01-11-2017, 06:08 PM
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To put a "The End" to this mystery for everyone, we had just put on aftermarket levers and while the clutch lever was fine, the brake lever wasn't milled deep enough so the plunger wasn't being fully released causing the piston to not fully release so every time he pulled the brake, it didn't release so after 5 minutes of riding, it overheated from constant pressure. Re-drilled the hole a bit deeper, we also had to grind a portion where the bolt goes through the lever as the angle of the lever wasn't quite to OEM specs, no more brake issues. Thanks again for everyone's input and advice!

Last edited by shaneman; 01-12-2017 at 04:53 PM.
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Old 01-15-2017, 08:44 AM
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That also explains why you were unable to back bleed the system. Master plunger engaged and there's no way for the fluid to come into the resevoir
Good work
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