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I've just had a problem come up with one of my XT disc brakes that I've had before on another bike and never got to the bottom of (even with LBS help).
The main issue is that the brake lever intermittently feels like it is "jammed on" giving no feel or modulation when braking. Normally, there's a reasonable amount of pull before the brake starts to bite and then some more until it is full on.
When the brake plays up, there's virtually no pull and the brake is full on.
From previous experience, bleeding the brake and even opening it up and giving it a damned good clean makes absolutely no difference as this problem.
While we couldn't confirm it, the LBS thought that some grit or something might be lodged in the chamber in the brake lever that you just can't get at, no matter how hard you try.
Has anyone else had a similar problem and, if so, what did you do to solve it?
On an another matter, I'm pretty sure the caliper has a lazy piston as well. It is really annoying!
Most likely a problem with the hose > the lever. This is the pinch point for oil flow and where if you were going to get a problem with lack of flow it would happen. Have you checked that out?
Yup. No kinks, no issues at all. It is really weird.
You've had the pistons out?
Knackered master cylinder - experienced it a number of times
Best fix is to replace the lever for about £30
Biggest downside i've found with the Shimano brakes 🙁
Sadly quite common on XT's of a certain age, the later ones were much better.
Warranty replacement is the usual fix.
I thought it was probably a master cylinder problem. Time to visit the LBS and get a warranty replacement.
Had a similar issue and it was dirt and grit in the master cylinder behind the lever blade. It is actually possible to completely strip and rebuild the lever assembly. That's what it took to remedy my issue.
I thought strip and rebuild was the Hope solution. With Shimano you just re-buy the offending component.