Home Mechanic: Shimano XT BL M775 Brake Levers

I recently picked up a pair of used brakes for my bike.  They are Shimano XT brake levers with XTR calipers, and they are pretty, albeit a bit beaten up. 

These brakes have AWESOME modulation.  Nose wheelies, anyone?

The only problem that I have is that the rear brake is not engaging well at all.  I thought that the issue may be with Shimano's "Free Stroke" adjustment.  I couldn't figure out what the hell it did, and why it wasn't really doing anything for me.  So I decided to take it apart.  This is partly a walkthrough, partly a tutorial, and mostly just a close look at what goes on under the lever on these brakes. 
The first thing that I did was take a look at what Shimano had to do offer in their tech help section.  That's where I got this document: Exploded view of XT BL M775. 

Looking around the internet, especially on forums and company tech support pages can be super helpful when you're about to dig unknowingly into an expensive component.  Here are the tools that I used:

The first thing I did was take off the plastic covers on top and beneath the lever blade.  Each have two fixing screws, and the top also has the free stroke adjust screw.  All are easily taken out with a phillips head screwdriver.  After I did that, it looked like this:

Top view.

Bottom view.

As you can see, I had slipped the plastic cover off the brake line junction with the master cylinder.  This was to gain better access to the lever axle fixing bolt, which resides in this hole behind the lever:

Directly to the left of the plastic cover in the picture.

Taking a 2mm allen key, I loosened it up (it doesn't need to come out).  The axle came out with just a bit of force from a 4mm allen.  NOTE:  the most difficult part of the operation was putting the axle back in.  If you are following these directions, read to the end before you take it out.
These are the parts that I had out up to this point:

Bottom cover w/ screws, lever axle, free stroke screw, and top cover w/ screws/

Removed lever.  I did not attempt to disassemble it any further.

At this point, I just took a good look at what was going on, cleaning out dirt and grime as I went.  The lever assembly has a plunger, a reach adjustment, and the cam rollers attached to the plunger.

From L-R:  Plunger, cam rollers, Lever axle fixing screw, reach adjustment body.

The caliper was more interesting.  After I cleared away most of the grime, I was able to discern some telling features that are hidden beneath the lever.  The piston is engaged by the plunger in a central concave divot.  It has a limit bar, which is the silver bar to the right of the piston in the picture.  Beside that is a return spring with a plastic bumper on top.  Mine is really worn, but I imagine that it had some sort of circular form in the distant past. 

You can see the lever axle fixing screw access hole on the right side of the picture.

My intent was to figure out why my brake wasn't getting any free stroke adjustment, so I put the free stroke screw in to see what it does.

What you see is the screw pushing down the piston.  Essentially, it's a limit screw for the piston.  By pressing the piston in, the idea is that engagement will be quicker the more it is screwed in.  I have already tried screwing in the free stroke screw all the way, and it is obviously doing what it is supposed to.  So, my problem must lie with the hydraulic fluid inside the system.  I plan on bleeding the brakes in order to add mineral oil and get rid of any air that might be in the master cylinder.  My investigation did not help solve my primary problem, but I got a few important things out of it.  I have a better understanding of how my levers work, one of them is freshly cleaned and greased, and I was able to fix a nagging hang-up sensation in the early lever stroke.  This problem was minor, but it has been bugging me a bit.  I looked at the lever's action, and determined that the hang-up occurs when the cam roller passes the bend in the dog-leg of its track:

Servo wave?  More like servo weak.

 As you can see from the picture, the roller has a slight groove from wear.  I was able to file the groove out slightly, so that hopefully it doesn't get caught in the corner, causing the hang-up I was feeling.

Putting the brakes back together was fairly simple, but the lever axle required some extra time.  The lever's reach adjustment axle hole is tricky to line up with the lever's.  I was finally able to put the axle all the way through by setting it in the lever body and lever, then pushing it against a counter while fiddling with the reach adjustment assembly.  Don't forget to re-tighten the fixing screw!

I'm disappointed that my brakes are not working perfectly, but it was fun to take the lever apart and learn some things.  Especially because I didn't break anything, I'll consider this a success.  My work isn't done, but I'll keep you filled in on how the bleeding goes.  In the meantime,

BE JEALOUS.