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I'm struggling with a possibly leaky XT front (M665 caliper, M765 lever) brake. I've had to bleed it pretty much once a week for the last 4 weeks. I always reverse bleed by draining it, then filling with a syringe to the bleed nipple until the reservoir is full.
There's no obvious leak, so there must be an air bubble trapped in the system. A friend recommended zip-tieing the lever to the bar overnight. So I did, and it was as solid as a rock this morning. By this evening, it's squidge-tastic again.
Should I top the fluid up, after the lever-tie thing?
Ive used the avid syringe on both ends create a vacuum and draw bubbles out method on other brakes .Trust me youl know if youve got a leak because the plunger will pull right to full extent without any back pressure filling the chamber with air.Its amazing how many calliper taps and lever flicks it takes to get all the pesky bubbles out even on a leak free set !!
anyone else?
As you've suggested above, you'll need to remove any air from the reservoir after the ziptie trick, leaving it in there is asking for problems later, but unless you're storing the bike upside down/hanging from a wheel, the air shouldn't find it's way back in unless the reservoir is near empty.
I had the same problem with M765 levers and M 765 calipers after a tumble.
No signs of a leak but there must have been air getting in, not oil out, if that's possible.
I tried and tried to bleed it ( only the rear was affected) and gave up after a few weeks of trying and sold them on eBay as seen, difficulty in bleeding etc...
Iwhen bleeding I'd get a big bubble out and think I'd succeeded. The next day they'd be pants again.
It might be worth re doing the hose connection to the lever, if you can be bothered. I suspected that was the thing that was to blame on my brakes (after thinking about it) as that was also the area that took the brunt of my tumble.
I think you need to decide if they are leaking fluid first. It has to go somewhere, so clean the brake and caliper, zip tie the lever to the bar, place a clean piece of kitchen roll under the caliper and master cylinder and leave overnight. Make sure you pull the cover off the hose connector btw.
Another idea (to rule out a faulty master cylinder) is to swap the hose over to the rear brake master cylinder, bleed and test.
Hth
Marko
cheers. I'm giving in and taking them to the shop to look at over the weekend.
With such a slow leak/issue id not be confident of a shop finding the problem. Take the pads out and check around the piston seals, mine had the same issue and i eventually spotted a very small amount of oil around one piston when you pulled the brake, this was obviously where the air was getting in. Unfortunately the calipers are not serviceable and i bought this particular brake s/h, annoying!
Well, if the shop confirms that, I've got no excuse not to buy new brakes 🙂