You know, it constantly amazes me that so many people are against using a disc brake on the rear of their bikes. I know some people have tried them but it sure seems like there are a lot more people talking bad about them than have actually tried them. Just so you know, I've run 2001 Magura Louise with standard black pads on both front and rear of my past three bikes (2001 Megamo 26 Pro, 2002 Echo ES2 and 2003 Echo ES4). I'll NEVER go back to rim brakes. I can ride wet or dry, hot or cold and never have to fiddle with tar or grinding anymore.
I would bet that most people who have tried a disc brake on the rear just didn't get it setup right. I'm not questioning their competance or anything, it's just that a rear disc brake requires quite a bit more finesse than a front.
The biggest problem is that most frames don't have the disc tabs perfectly aligned. We all know how finicky rim brakes can be about pad alignment. Well, disc brakes seem about 10 times more sensitive. At first, I was able to get by decently well with some creative work with the 0.2mm spacers. But, once I finally got the disc tabs evened out using a Hope Spot Facing Tool, my rear brake has improved immensely. You cannot imagine how much better a rear disc brake works once it is aligned properly.
The second thing is bleeding. Since disc brakes work at a much higher pressure than hydraulic rim brakes, they are much more sensitive to air in the lines. Even bubbles only the size of the tip of a sharpened pencil have a noticeable effect. I've recently got my hands on the new blue "Royal" blood from Magura. It seems to be just the slightest bit less viscous than the green and so air bubble can escape faster.
Before you even start thinking about bleeding, take out your fluid and leave the bottle sitting still for about 20-30 minutes to let the bubbles generated by movement rise out. When you are ready to begin the bleeding, try to be very careful not to jostle the fluid around, creating new bubbles. Follow the normal bottom-up bleeding procedure outlined in your manual.
After you have filled and drained the master reservoir 2-3 times, push enough fluid into the reservoir so that it just barely starts to overflow the edge. This is VERY important! Any space left between the top of the fluid and cap will cause the brakes to feel spongy and reduce the pad travel.
Lastly, before removing the syringe, push on the syringe plunger and work the brake lever back and forth to move fluid in and out of the system. Take careful notice to see if any bubbles come into the syringe line and make sure to get them out of the brake system.
For those few of you out there that are running a rear disc brake, try these steps and write back here to tell everyone your results. I suspect you will be amazed how much your brakes will improve.
If you get a frame that is disc capable you are choices are pretty damn poor. Echo to my knowledge does not support disc brakes anymore. PX is the same way. So you real choices are Megamo and Brisa. Also not everybody likes the way the disc locks, and by that I mean the springy sensation you get when the hub stops causing the spokes to flex a bit.
