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Rear Brake Sticking

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Hey guys, I'm hoping you'd have some insight into this recurring issue I've been having with my 2015 300 XC-W that I purchased 2nd hand earlier this year.  During some of my rides, the free play of the rear brake pedal will gradually decrease during use to the point where the rear brakes begin to drag and in a few cases causes the entire rear wheel to lock up.  I always try to maintain some additional free play over the manual recommended 1/4" free play (Id say Im closer to a half inch, maybe even more then that).  I am a relatively heavy rear brake user, but I try not to drag it much, and tonight it locked up in about 10 minutes of riding around a course right out back and I was trying to practice keeping off the rear brakes and they still heated and locked up.  

When they do lock up on the trail, I increase the amount of free play in the rear lever by losing the lock nut and spinning the cylinder plunger adjustment screw until they finally loosen up give it some additional free play.

I've changed the fluid out twice this summer, I'm running the Motorex 5.1 fluid.  I even lowered the amount of fluid in the rear reservoir so there would be additional room for fluid expansion when it heats up.  It seemed to be working fine for the last 2 months, it hadn't locked up or dragged since July, until yesterday.  Yesterday and today it has been unusually hot and humid here, I'm not sure if atmospheric conditions could have an effect on this?

Any hints and suggestions to cure this problem?

My old 2000 EXC 200 never had  this issue, I used to boil the rear brake fluid and they would fade away on that bike...but I never had this lock up problem.

This 300 is an eastern woods weapon, man what a bike!  I'm finally starting to get it setup to where I really like!  All I need to do is fix the rear brake! Thank you very much in advance!

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On my 300 xcw I had the same thing happen to me, it turned out to be a bad banjo bolt.  Using a hand bleeder I noticed no matter how hard I tightened the bleeder screw would suck air bubbles.  I replaced the banjo bolt and crush washers and have been good since.  Very strange but was a relatively inexpensive and easy fix.

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Have you stripped the rear calliper? It 'floats' on the 2 mounting pins, one fixed to the mount and the other on the calliper. These need regular cleaning + the rubber covers - we don't put anything on them to attract muck but they still do. Also you could very carefully pump the piston out a nad (not past the first seal obviously) and spray copious amounts of brake cleaner down around the piston and blow dry.

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12 hours ago, KTM_rider_EK_17 said:

https://www.rockymountainatvmc.com/p/1193/38999/All-Balls-Rear-Brake-Pedal-Rebuild-Kit?v=11987

 

SORRY, SORRY. Thats the wrong part. Its actually this one but doing both would be better and assure that you don't have this problem again.

Exact same symptoms have been happening for me, I have a hard time thinking it is anything hardware related in the front as the pedal moves freely.  I have increased freeplay to more than I care for.  I will try cleaning the caliper as others have mentioned.

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If the push rod is adjusted so the fluid does't flow back into the MC it will do what you are saying.  Had it happen to me.  If the fluid has some moisture in it, expansion will happen more than usual.  If it doesn't push into the MC it will make the brake go on or at least make it so there is less free play.  As the brake goes on it gets hotter then locks up.

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I have my doubts its hardware related as well, the pedal return spring, push rod, and related hardware all seem to be in fine condition.  The banjo bolt suggestion is pretty interesting.  When I changed my fluid out twice this summer, I noticed something odd.  When I tried to bleed the brakes the traditional way (pump the brake pedal, hold, open bleeder, close, repeat) I could not get the system to build pressure at all.  I eventually reverse bled the system using a syringe I had to bleed the clutch system on the old EXC 200 and that seemed to work well and the brake action returned pretty quickly doing that.  I'm not sure why traditional bleeding didn't work.

Another instance this summer, I replaced the brake pads after wearing them down to just about nothing on a long, wet ride, and on the first ride following the brake pad replacement, the rear brakes lost all pressure shortly into the ride and I had to make do with only front brakes the rest of the day.  Upon inspecting the system after the ride, the reservoir had sufficient fluid in it and I couldn't find any evidence of anywhere where air would be entering the system.  Again, I needed to reverse bleed the system to get the brakes to function again.  Possibly a bad banjo bolt or copper crush washer would cause a slight air leak into the system?  

As for the push rod adjustment, how can you tell if the rod is adjusted to the fluid can travel back into the master cylinder?  I mainly have been adjusting the rod to maintain enough free play in the pedal, I haven't taken account adjusting to make sure there is enough backflow of fluid.

I did strip and lubricate the pins on the caliper and it seems to be floating well.

Thanks for the responses, I really appreciate it!

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If you can push the caliper into the disk with your hand or even a knee and the caliper piston moves it is flowing back to the M/C. 

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I tried pushing the caliper in by hand this evening after washing the bike, nothing.  I then increased the amount of pedal free play and it must have opened the passage to the master cylinder/ reservoir because then the caliper began to move quite freely by hand after the adjustment.  The pads have worn considerably since the last time I adjusted the fluid level and the fluid level was quite low in the reservoir when caliper was extended far enough so the pads are contacting the rotor. As I pushed in on the caliper the fluid level rose just about to the top of the slight glass....where I had set it last time I changed the brake fluid.

I also checked the rubber compression bladder that hangs down from the M/C cap, I gave it a good squeeze and some nasty looking stuff came out of the vent hole at top followed by some clean fluid, the vent hole may have been a bit clogged too but now its breathing freely.

I'm planning to replace the pads, probably keep the brake fluid level about the same in the reservoir, then adjust the free play to a point where the caliper can freely slide back and forth without having a massive amount of free play.  Then hopefully its ready to rip!

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I had a similar issue on the weekend, check your brake line isn't rubbing on you muffler/exhaust. I found mine was touching and must have been boiling or heating the fluid and pressurising. It had the exact same symptoms as yours. I held it down with a zip tie and that seemed work for the remainder of the ride no more brake dragging.

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Probably the best way to set the level is to take the cap of the MC, push the caliper piston in all the way.  Fill it above the sight glass and below the threads then put the bladder with the cap back on and it will probably overflow to the maximum height.  The fluid should be changed.  If it was causing pressure when hot it is most likely saturated with moisture. 

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