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Today while riding down a long and technical trail, I noticed my front brake lever seemed to lose its free play and the lever seemed to stiffen up. I continued descending for another 5 minutes and the front brake began to engage automatically. Within a minute, the front brake was locked firmly on the disk. I couldn't turn the front wheel. The lever could not be pulled back.

This continued for about 5 minutes until things cooled down where gradually the pressure on the lever and disc relaxed and I was able to freely spin the wheel. 

I figured perhaps my finger was slightly riding the brake and causing friction and heat. I consciously finished the remaining descent trying to minimize the front brake. 

I rode the rest of the day without incident until i was riding home at the end of the day on a logging road when suddenly it seized up solid again. After it cooled, increased the lever free play to about 3mm which seems ok for now. I have a 2009 Beta Evo with stock brakes I think.

Anybody seen this before? Any recommendations?

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Increasing lever free-play may have fixed it. 

At some point in the bike's life did you top-up the fluid and then later replace the pads?  Trials reservoirs are small -- maybe there was not enough room for thermal expansion of the fluid.

In any event, I would check for pad wear and evidence of them binding in the caliper; exercise the piston travel with the pads removed and clean everything; perform a fluid change.

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I know it sounds daft but have you checked state of your pads.i had same problem kept backing off adjuster cos they kept sticking on,checked pads mine were down to the metal replaced now no problem

 

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Hotweather can be a problem with expansion, crack open the bleeder to get you home, then assess whats really going on, on enduro bikes a warped/bent disc can cause this issue....there are a lot of scenarios with this as previously stated.

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Biggest chance to fix it: Change fluid.

Check caliper piston returning (*) them with fingers. It should be hard but same effort on all four.

* when returning brake calliper pistons for replacing pads or anything else, always clean pistons first. Or you will inject dirt into system.

* cut a small chamfrer on new pads. The idea here is ease disc placement when reinstalling wheels. This way you will not need to return calliper pistons every time you deal with wheel maintenance.

Edited by cascao
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Perhaps you didn’t have enough free play in the system. When I got my bike and rode it for the first time I made a complete t*t of myself by falling off in the car park in front of a crowd of people. Having come from cars and karts where I was obsessive about having a short travel and a rock-hard pedal, I over-adjusted it, and it “pumped up” and locked. The simple solution was to back off the lever adjustment a touch. Trials bike brakes are so powerful that they deliver a nice hard feel without too much fiddling.

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Have Braktec on my Scorpa and had an issue just like you're describing.  Noticed the brakes getting hard from excess heat on one ride and had to hit a stream to cool them down; only happened on long downhills but brake action slowly got worse and it was bugging me.  Flushed, bled, pads etc, no real difference and the only thing I could think of to cause this was the back bleed hole to the master wasn't open enough to release pressure on the pads when off the brake.

Took the master apart; there's a washer that goes around the stem on the master piston that is held in the bore with a circlip and it keeps the piston from backing all the way out.  Turns out that washer had a warped depression on it right around the ID, this caused the washer to have a bigger thickness (depression held the piston deeper in the bore) which was causing the back bleed hole to be partially blocked.  Weird part was that since it was oddly shaped the depression only had that effect when in certain positions.  That led the brakes being fine for some of the time and overheating at others; take a look at yours.  I just hammered the washer flat and put it back in, no problems since.

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I've noted that Mountainbrad said that he lost the freeplay in his brake, indicating not enough mechanical freeplay in the lever adjustment. 

After a ride, with my bike back home I noticed brake fluid weeping from under the rubber diaphragm of the reservoir's cap. No big deal, I just tightened up the screws a bit. The next time out on the bike the brakes started to drag, even with mechanical freeplay being felt. I ride in similar topography to Squamish, nearby on Vancouver Island, where we experience elevation changes of over 1550 m or 5100 ft. The solution was to open up the venting channel cast into the underside of the cap. I widened it carefully with a hacksaw blade. Now the cap vents to compensate for air pressure changes up and down the mountains, as well as keeping the fluid in the master cylinder. Even if you don't ride up mountains, seasonal air pressure and temperature changes could affect the brakes with a cap screwed on too tight.

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