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Disc break stuck on
As the title suggests, went for a ride yesterday and about 10km into my ride my rear brake stuck on, made pedalling very difficult 
Couldn't free it any more than half a turn, tried forcing the pads back but it made no odds. It eventually locked up completely again and my legs couldn't take any more so I took the pads out and completed the ride with just the front.
So two questions, what caused this and how to fix it? Would be nice to have fixed it whilst out on the trail.
Brakes are Avid Juciy 5's on Treck EX8
Cheers
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Sounds like the fluid cannot get back into the reservoir for some reason - either because the master cylinder piston is not returning far enough or the reservoir is overfull and the hotter weather is expanding the fluid and causing the pistons to move out and lock the brake. Is the lever very hard with no freeplay? I'd try removing the master cylinder lid first being careful to have it orientated so as not to lose all the fluid.and see if it is a bit full. Off course it could also be a frozen caliper piston/s. Let us know how you go - I have some other ideas if that doesn't work.
The brake lever was fairly hard with little play in it, trying to pump the brake seemed to help slightlly but it didn't last. I'll try and check it out tonight.
Cheers
Maybe a lack of use
Well it is easy to prevent anyway, bleed your brakes.
The DOT fluid is hygroscopic so over time it absorbs water into the system. On a hot day the water expands and causes the brakes to lock on.
If this happens out on the track you can normally just open the top cap of the lever and it will normally let out enough fluid (and then close it again)that you should still be able to use brakes and ride home...
If it is very bad you may need to press the pads back into the caliper with the top cap off to push out enough fluid to allow them to work again.
So the message is 'don't go out to play in the heat' as Brian has already alluded too the MTB doesn't see enough action as it is. 2 rides in 3 weeks = dented forehead and numerous other niggles and now a locked on brake, best stay in doors or on the road bike
This happened to me also. Dont open ( Master cylinder cap) top cap you may get air in the sysem, there is a bleed screw on (its a torx scew) top cap. pull brake lever in gently then undo scew slightly to bleed some oil out tightin scew up then release brake lever to stop air entering system this will allow some expansion room in reservor. One of the symtems of brake lock on Juice 5 is a over full reservor causeing pressure build up, especialy on hot days. Mine used to even weep oil out from under cap on realy hot days out in direct sun light.
Breaks locking or dragging seems to be a very common problem with Avid breaks. My sons and my bike both have Avids, as well as a friend, and all of us have issues with breaks locking or dragging. All bikes are 2010 models and the problem was there from new.
The problem seems to be corrected by bleeding a small amount of fluid. For all 3 bikes the hydraulics seemed to be overfilled. My breaks still seem to drag occasionally and then be fine a few minutes later.
I will try bleeding but the other bikes seem to new to require this.
Either way, I am sure every time someone on a forum spells brakes/breaks wrong, a mute mountain biker has a tree fall on them in a forest.
Andrew
of a typo, not sure that particular mountain biker will be of much use to fix my 'brakes' then.
Breaks (what's wrong?) jammed up on both of my bikes (avid 5 & 7's) when in the back of the car. Bled them using the Avid bleed kit.
Follow instructions and don't lose the torque head screws. Works perfectly now (until next time which is about 12 months). Make sure the pads/pistons are wedged open using the supplied wedges.Should be plenty of play at the lever.
Don't know what causes it. One had air bubbles the other 3 didn't.
Mine are 2006 vintage. Surprised the problem has not been sorted by now.
It is most likely air in the system that causes the brake lock up on hot days. Brake fluid is designed to handle high temperatures which occur every time you use the brakes. The high atmospheric temperatures don't have much effect on the expansion of the brake fluid, but they will expand any air in the system. The air may be trapped up near the levers which is why it is not affected by high temperatures caused by normal brake operation.
It seems to be a problem more prevalent on avid brakes.
Elixers seem fine but every juicy I've seen gets this problem sooner or later.
Every system needs room for expansion, whether it has air in it or not. Juicys are always overfull from the factory so as they expand or get a bit of air in them it's got nowhere to go.
Just let a bit of fluid out of the reservoir. You may want to bleed them too, It's always a good thing, but that's not what's causing the problem.
juicy junk
FWIW I've never had any problems with mine Three's and Five's
They've been round long enough so any problems should have been fixed or specific advice available to avoid the problem.
I know how to bleed brakes and I've never had this problem before. Air is getting in somewhere and I don't now where.
This time I've bled the system using the Avid bleed kit as instructed, tightened up the torx head screws using a bigger handled key, left 10mm of travel in the levers before takeup hoping the problem is solved.
No trouble-shooting section in the manual. No advice as to the amount of free travel in the levers before the brakes should engage.
Not good enough.
I won't be buying any bikes with Juicys again and I'm wary of Avid in general.