At the end of my last ride the back brake started rubbing hard on the disc. It almost feels like the brake cable had popped out of the lever. The bike is relatively new to me, and I am not very familiar with disc brakes. I can take it to the shop, but I wanted to be able to fix myself in case it happens again. Any ideas on how to fix, or what caused this? It is a Specialized Epic with Avid brakes.
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Ensure the axle is properly placed in the dropout and the quick release is firmly closed. If it's even a little loose, the axle can shift a bit and cause the rotor to be misaligned with the caliper.
If your brakes are Avid BB7: I use those on my DH bike. I have had the opposite issue: the brake loosing grip ar the end of the ride.
In my case the outboard adjusting knob had been missplaced so it alowed the internal adjunsting axle to spin a little. I repositioned the know and noticed it could be easily turned by rubbing it with other things (Speccially while a bunch of bikes piled together being carried to the top of a DH track). I just have to be careful with that.
However, there are other factors that can affect these and other models of brake, mainly misalignment. Check whether the caliper is centered on the disc and thati it is holding the pads parallel with disc surface. The easiest way to check (at least for me) is to place the bike on a shadowy place and look throug the caliper's gap placing a flashlight on the other side. Look alongside the disc surface (so you just see its border) and examine the disc-pad gap. (As other answer mentions, first check that the entire weel is properly attached, aligned and tightened)
It should be aproximately 0.5 mm on the outboard pad (this is the one that actually moves when you actuate the lever), For the inboard pad (the static one) it should be as close as possible without touching the disc.
Aside from the caliper being properly aligned and the holding bolts being properly tightened, that should be enough. I'm assuming you have no cable issues here, I don't know your particular bike but chet if no part of the cable is hooked in a part where it shouldn't and that all housing stops are properly seated.
A relatively easy way of aligning the caliper (actually described more precisely on avid manuals) is to loosen the holding bolts just enough for the caliper to move freely. Squeeze the corresponding brake lever and while holding it down, tighten the bolts again. Check and repeat if necessary, but ussually one or two attempts are all it takes. The only one thing some manuals do not mention is: Make sure that anything is pushing or pulling the caliper! Sometimes the cable-housing assembly pulls the caliper out of alignment when this procedure is performed (it happend to me on a 2003 SantaCruz Bullit) I just pull the housing to a position where this effect is lessened.
The mechanic bleed the lines which fixed the problem. They really didn't know what caused this -- they implied it could have been an issue with manufacturing or assembly.