I've got a friend's bike that I'm trying to diagnose a problem on - XT (M765) dual control shifters, hydro brakes, XT (M775) calipers - the rear brake gradually got weaker and weaker to the point the lever was all the way back to the bar and it wasnt doing much stopping. When I took a look at it, the fluid reservoir was almost empty and the rear caliper was full of dirt/crud. I cleaned off the caliper (pads out, washed off, pumped the pistons out and cleaned with a q-tip and alcohol), added fluid and bled the brake. Lever pressure came back, but they would howl a bit with really pull on the lever and the stopping power was better but not great. I tried another set of pads and a different rotor on another wheel, and no change. Now that the bike has sat most of the winter with muddy trails, the lever comes back to the bar again and the stopping power is still poor with a lot of noise.
Thoughts here? The low fluid in the master concerns me, as it had to go somewhere - the pads were not very low, so where did it go? Leaking out of the caliper seals? The bike is a few years old and I dont think the fluid was ever flushed on it. I checked with the LBS and they said shimano doesnt sell rebuild parts (caliper seals, master cylinder seals) so if we're buying parts what should I start with? The levers are cheap on ebay, the caliper a little less so, but I'd rather not replace parts that are working.
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