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I've got these on my 29er and the front brake has developed a mind of it's own. Basically, the bite point can be very inconsistent. Usually when you're tanking it downhill, go for the brakes only to find you can practically pull the front one back to the bar. Which can get a bit worrying.
Now a couple of things. I don't drag the brakes, no really I don't. It only happens sometimes as I've tried to recreate it on purpose but can't seem to do so.
Aside from a full bleed, what can I do? I'm not convinced the bleed will resolve it as otherwise they're superb. I know some shimano brakes are known for this but otherwise I really like them.
Any recommendations?
Check for weeping oil and check whether this disk is centered and not warped.
Other than that, it's a known issue with Shimano brakes. Lot's of chatter about it, I'm moving back to Hope because of it.
I'm thinking of getting some Guide RSC's which I'm a big fan of. Good point about the potentially warped rotor but pretty sure' it's OK. Will make a point of checking that.
I've got a Zee and an SLX that do this, whereas my other half's XT are great- it's annoying as hell! Just swapped for Guides, much better so far but the rear brake seems to come gradually closer to the bar, so either it's not great for bleeding (seems fine every time I do it) or the reach adjuster is self-winding. Going to get a dot of tipex on it to see if this is the case.
Check your calliper to see if you have a sticky piston on one side
I think its the piston seals - although I'm not exactly sure why
I had the same problem with some XTR m985 brakes. Bled them at least 5 times in numerous different ways. Each time black fluid came out (not sure if this is connected) and each time inconsistent lever feel. The brakes would pull all the way back to the bars on the first stroke and then be fine on the second (if you pulled it quick enough). So this meant in order to brake you had to pull the lever twice in quick succession. A temporary fix is to take the wheel out and pump the brake till the pads sit closer to the rotor ( A process that should happen automatically as the pads wear down). This works but only till your pads wear a little more then you have to do it again.
This behavior led me to suspect the piston seals as they are responsible for returning the slave piston to the correct position and for adjusting for pad wear.
So I bought a set of SLX calipers from chain reaction and swapped over the piston seals and pistons (they are the same).
So far the brakes have been fine - adjusting for pad wear as expected
I could see nothing visibly wrong with the seals that came out but perhaps their was some invisible difference?
who knows
I had some contamination to the fluid in mine. Bite point was inconsistent and turning the bike upside down for any length of time meant the brakes felt like someone had reset the pads. On bleeding there was clean fluid at both ends and with the bleed block in it felt solid.
Tried most of the recommended solutions bar overfilling the system, some of which helped to a degree but didn't fix it.
In the end, after turning the bike upside down showed the same problem again, I thought that whatever was causing it must now be in the lines and pushed a load of fluid through from the caliper up to stop it sinking. Got a load of black fluid coming out at the top. Pushed all of that out and it seems to have solved the problem.
Best guess for me is some contamination in the fluid that sits somewhere a normal bleed doesn't get it. May be a symptom of iffy seals but seems to have solved it for the moment.
Air. it's always air in the system. IMO as the owner of two sets of these for the last 5 years, these brakes are not as easy to bleed as shimano suggest, and they have dicey lever seals. When they're bleed right, they're great, but even more I don't get it right all the time.
ndthornton is correct.
My xtr 980's suffered from it and got progressively worse.Replaced the piston seals (from a another broken caliper) .Problem solved. Replaced both calipers with some new xt ones (it was nearly the same price as just buying new pads!!) and the are totally fine.
To be fair they were 6+ years old and had been fried a couple of times so not totally unexpected.Had more issues with hopes over the years.
Thanks for the helpful replies. Will try and spend some time this weekend having a look into it.
if theyre less than 2 years old send them back to shimano under warranty replacement
if theyre less than 2 years old send them back to shimano under warranty replacement
Hadn't thought of that!!