[SteveOr]

One of the pistons on my rear 2003 XTR disc brakes leaks oil when braking and it is compromising braking performance. When I apply the brake lever the piston gives off a sizzling type sound and minute flecks of oil spatter out along its side. Prior to that I just changed the oil, bled out the air, etc., etc. according to the Shimano manual (I did this because I accidentally leaked fluid from the other piston when I was trying to find a way to stop a rubbing noise). The other piston works fine but the affected one doesn’t travel as much. I can still stop pretty well for now but it is pretty mushy compared to the stiff responsiveness of the front brake. I wanted to know if anyone else has tackled this problem so I can avoid another fruitless quest to my local bike shops. By the way I used regular mineral oil instead of Shimano's. Is that a big deal?
Thanks, Steve

 

[oldfart]

Maybe you have a lazy piston and combined with pads that are almost worn out, the piston which won't retract all the way is too far out? New pads and lubing the seals of each piston might help. Remove the pads and squeeze the lever to push out the pistons. Use a wrench to physicaly hold in the pad that wants to come out the most to force the other out then drip some mineral oil onto the piston edge, push the piston back in, pump it out and push it in several tiems for each piston. I don't think there would be any problem with using a regular mineral oil as oposed to Shimano mineral oil. DOT fluid would be incompatible of course and destroy the seals pdq.

If its getting mushy you probably sucked in some air when you blew out some fluid. I would be surprised if a seal was damaged. I would think if it was a damaged seal that it would have been damaged from day one.