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Like many here I have XT M785 brakes that seem to gradually self-contaminate thanks to a barely noticeable leak. I'm getting irritated with the hard "burn off" mile or two required at the start of each ride to bring the brakes back to life.
I'm pretty sure the front leak is from around the piston (less certain about the back).
Has anyone tried the replacement pistons below? If so, did it cure your leak? Did you replace the other 2 seals when splitting the caliper? Where did you get those from?
I've seen all the threads saying just buy new a caliper or brakes. Cash wise yeah that makes sense (e.g. M7000 SLX at £25 incl. pad makes the effective caliper cost £10-£15 vs that piston kit). Splashing out on e.g. Magura MT8 carbon is also tempting!
On the other hand I dislike chucking stuff away and would prefer to repair as generally I've been pleased with them.
How old are they? I had some Deore's that leaked and they were replaced under warranty after 20 months.
I'd just buy new calipers - at the least then you know it will fix the problem.
Slackboy I bought them second hand a couple of years ago so no warranty.
That eBay piston kit is actually only 1 piston + 1 seal, i.e. a pair is ~£13 (and there's no way I'm stripping the caliper just to replace the one side I can see leaking), which makes a new caliper a no-brainer cost wise. If someone knows a cheap source for the square piston seal I'd be vaguely tempted though.
There is a thread on here about the leak potentially being caused by an O ring at the caliper join too, isn't there??
Poopscoop yup I saw that thread. However after carefully cleaning everything I can clearly see the front leak is from a piston. Back still needs more observation.
Note to self: eBay link for the little o-rings (between caliper halves) from earlier thread is still valid -- https://www.ebay.co.uk/itm/141455807147?_trksid=p2060353.m1438.l2649&var=440603744128&ssPageName=STRK%3AMEBIDX%3AIT
Still looking for specs of the square piston one.
In your situation I think I’d throw the Shimano brakes away and get some 4 pot Maguras with the short / 1 finger lever.
Joebristol- 4 pot all round would be overkill for my recent riding! MT Trail are an option though.
The M785 are easy to bleed and I can share spare pad cache with the road 785s on Arkose.
I don’t think 4 pots are too much for anything really. I’ve got sram guides on both my mountain bikes and they’re great / not that heavy and modulate well.
I have 785’s on my road bike - they’re powerful but are a bit more on / off and haven’t got a great reputation for reliability (although mine are coming up for a year with no issues yet).
I’ve just picked up a sram code r to put on the front of my big fs bike to see how that is, but I haven’t fitted it yet.
I'd get the SLX calipers, then you'd get 2 year warranty on those as well.
Will msg you as well.
There easy to bleed, handy because theyre permanently faulty in my experience of a few sets of my own plus numerous riding buddies. Get rid and get sram or another brand that you barely have to touch.
Thanks Joe. I've heard a lot about guides but no experience. Will watch out for your thoughts on the codes. I've basically decided to just get a new M7/8000 caliper as granny_ring suggested. In the meantime I have given the leaky part a good clean with white spirits and might try sticking some absorbent foam round the piston.
My road 785s have been flawless since I got them in 2016- only pad changes and no bleeding. Agreed about the modulation but I've got used to it and don't mind.