Stuck front brake 2012


12 replies to this topic
  • BRX-WRX

Posted March 10, 2014 - 04:43 PM

#1

Ryder is bringing a 2012 wr450 over for me to figure out what making it stick. Says he went to a 2hr race Sunday and when he got home and rode it to get some mud to fall off his bike his front brake stuck and put him down. Says his lever would not budge. I told him to loosen his cap to see if that helps, said he got it to work but locked up again after awhile. Any known problems on this ? New leftover bike. Thanks MB    I will let you know what I find latter. Hoping for a head up.



  • WALKINGWOUNDED

Posted March 10, 2014 - 05:06 PM

#2

Not directly familiar with said bike, but look for mud built up around the piston bore in the master cylinder.  If the piston can't return all the way to its rest position, that will keep fluid applied to the caliper.  As it heats up, the fluid can't return to the reservoir because the piston isn't fully seated, and the internal return hole is blocked.  It will make for a very hard lever.

 

Also make sure his brake lever isn't adjusted to close to the master cylinder piston, or bent and doing the same.  This will also keep pressure/fluid applied to the caliper.

 

If you want to test and see where the pressure being applied is coming from, ride the bike (slow and careful) until you feel it starting to drag.  When it does, open the bleeder on the caliper.  If a little fluid spurts out, and the wheel frees up, the you know its a hydraulic problem.  If it doesn't free up, then it's a mechanical problem at the caliper.



  • Krannie McKranface

Posted March 10, 2014 - 05:16 PM

#3

The vent on the master cylinder cap (on the bars) is plugged up, and/or/+ your disc is bent, causing the pads to transfer heat to the brake fluid, causing it to expand enough to lock up. 



  • BRX-WRX

Posted March 10, 2014 - 09:53 PM

#4

I could not find a reason?  Owner took the pads out to bring it over. I vacued the old fluid out and started over. Disc strait , line 's like new, no kinks or any signs of damage. Refilled and then started working the pucks in and lever back out a dozen or more times no problem. the only thing I noticed was he had the newer acerbis bark busters that taper in from the side of the bar more than the original and when I pulled the brake lever in it would slightly snag on the end of lever, not leting ther lever back out all the way. Readjusted perch in . Not sure but I think that is what happened and he did not notice what I saw.



  • Monk

Posted March 10, 2014 - 10:05 PM

#5

I could not find a reason? Owner took the pads out to bring it over. I vacued the old fluid out and started over. Disc strait , line 's like new, no kinks or any signs of damage. Refilled and then started working the pucks in and lever back out a dozen or more times no problem. the only thing I noticed was he had the newer acerbis bark busters that taper in from the side of the bar more than the original and when I pulled the brake lever in it would slightly snag on the end of lever, not leting ther lever back out all the way. Readjusted perch in . Not sure but I think that is what happened and he did not notice what I saw.


Could have crashed which moved the barkbuster inward not allowing the brake lever to return to normal....

  • mebgardner

Posted March 11, 2014 - 06:54 AM

#6

I could not find a reason?  Owner took the pads out to bring it over. I vacued the old fluid out and started over. Disc strait , line 's like new, no kinks or any signs of damage. Refilled and then started working the pucks in and lever back out a dozen or more times no problem. the only thing I noticed was he had the newer acerbis bark busters that taper in from the side of the bar more than the original and when I pulled the brake lever in it would slightly snag on the end of lever, not leting ther lever back out all the way. Readjusted perch in . Not sure but I think that is what happened and he did not notice what I saw.

 

Like KoolAid indicated, the Master Cylinder vent could still be blocked and the system would still allow you to perform all the maintenance steps you've just done, and remain blocked, and appear to work normally.

 

You do not say whether or not you looked at this.  If not, have another look.



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  • grayracer513

Posted March 11, 2014 - 11:17 AM

#7

The brake is staying locked because the master cylinder piston is not being allowed to return to its fully returned position.  This holds the fill port of the cylinder closed, which has no effect on anything at first.  But, when the fluid heats up, it expands, and because it cannot escape up and into the reservoir, it applies pressure to the brake.

 

Possible causes of this are replacing the lever with one made for an earlier model, reversing the plunger rod that goes between the lever and piston, hand guards impinging on the lever, failing to let it return, or some similar condition.

 

The only way for the vent to cause this is if the cylinder is filled to the top, then closed up with the expansion boot completely collapsed. 



  • mebgardner

Posted March 11, 2014 - 12:00 PM

#8

I'm back to school, again. Thank You!



  • mebgardner

Posted March 12, 2014 - 07:17 AM

#9

The brake is staying locked because the master cylinder piston is not being allowed to return to its fully returned position.  This holds the fill port of the cylinder closed, which has no effect on anything at first.  But, when the fluid heats up, it expands, and because it cannot escape up and into the reservoir, it applies pressure to the brake.

 

Possible causes of this are replacing the lever with one made for an earlier model, reversing the plunger rod that goes between the lever and piston, hand guards impinging on the lever, failing to let it return, or some similar condition.

 

The only way for the vent to cause this is if the cylinder is filled to the top, then closed up with the expansion boot completely collapsed. 

 

I'm brake ignorant. Would someone please tell me what the vent is for, and what are symptoms if it is clogged (I thought that problem could occur easily).



  • grayracer513

Posted March 12, 2014 - 07:30 AM

#10

If you want the fluid in a reservoir to drain out the bottom into, say, a brake system, the reservoir must allow air (or something) to enter the reservoir to fill the void left as the fluid drained.  Likewise, if the fluid is to be allowed to move back up into the reservoir as it heats and expands, a path must be provided for the air displaced by the incursion to escape.  The "vent" on your master cylinder is nothing more than two 1mm notches in the back edge of the cover.  Nothing fancy.  Very difficult to really "clog". 

 

But since it's extremely important to keep the outdoors out of the brake system, the master cylinder cover is sealed with a gasket that has a bellows built into it so that the vent air, and anything that might come with it, is sealed out of the brake fluid even while the bellows expands and contracts to accommodate the changes in fluid level that occur. 



  • mebgardner

Posted March 13, 2014 - 11:08 AM

#11

If you want the fluid in a reservoir to drain out the bottom into, say, a brake system, the reservoir must allow air (or something) to enter the reservoir to fill the void left as the fluid drained.  Likewise, if the fluid is to be allowed to move back up into the reservoir as it heats and expands, a path must be provided for the air displaced by the incursion to escape.  The "vent" on your master cylinder is nothing more than two 1mm notches in the back edge of the cover.  Nothing fancy.  Very difficult to really "clog". 

 

But since it's extremely important to keep the outdoors out of the brake system, the master cylinder cover is sealed with a gasket that has a bellows built into it so that the vent air, and anything that might come with it, is sealed out of the brake fluid even while the bellows expands and contracts to accommodate the changes in fluid level that occur. 

 

Thanks very much, thats clear to me.

 

May I rebut that my original point seems, to me, to be valid: If the vent *does* clog (and does not allow the gasket / bellows to expand / contract), and air pressure within the (clogged, now airtight) bellows cavity attempts to prevent the bellows from compressing, *then* the OP would get the same result, a "stuck brake" as the heating / expanding brake fluid works against the airtight system.

 

Yes?



  • grayracer513

Posted March 13, 2014 - 12:26 PM

#12

No.  Assuming the piston will return unhindered to its at rest position, what a clogged vent would first do is prevent fluid from filling the void caused by pad wear.  The fluid would want to flow down into the system, but would create a vacuum above the unvented bellows, and this would lead to a loss of hydraulic pressure when the brakes are used, excessive lever travel due to the pads inability to advance for wear.  

 

If the system does perform normally at first, then the expansion from heat would force some fluid back upstream.  That could compress the air volume trapped above the bellows to as much as 25 psi before any brake drag became noticeable.  Only if the entire volume captive below the blocked vent were liquid would you get the same result as with the non-returning piston.   If the reservoir were overfilled to the point where there is no airspace above the bellows, or so nearly so that the bellows would become completely compressed against the top of the M/C cover, then you would start to see it add pressure to the brake.



  • mebgardner

Posted March 14, 2014 - 11:51 AM

#13

No.  Assuming the piston will return unhindered to its at rest position, what a clogged vent would first do is prevent fluid from filling the void caused by pad wear.  The fluid would want to flow down into the system, but would create a vacuum above the unvented bellows, and this would lead to a loss of hydraulic pressure when the brakes are used, excessive lever travel due to the pads inability to advance for wear.  

 

If the system does perform normally at first, then the expansion from heat would force some fluid back upstream.  That could compress the air volume trapped above the bellows to as much as 25 psi before any brake drag became noticeable.  Only if the entire volume captive below the blocked vent were liquid would you get the same result as with the non-returning piston.   If the reservoir were overfilled to the point where there is no airspace above the bellows, or so nearly so that the bellows would become completely compressed against the top of the M/C cover, then you would start to see it add pressure to the brake.

 

OK, thanks, got it.






 
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