I have a set of XT brakes on my main bike. Had them about 2 years and have been fine up until now. I havent rode the bike much recently, once in 4 weeks. When I went to use it today the back brake felt very spongy, almost pulling to the bar. I used it for a bit but it became a little dangerous so decided to give it a bleed, it felt marginally better but still to spongy despite feeling solid when i tested it with the bleeding block in. Changed the pads for new ones and it felt good again. Rode a little more and it started to feel a little bit spongy, but still usable. When I got home i put the bike on its back wheel to wheel it through the house and now the brake feels totally dead, nothing there, worse than before. Whats going on and why would putting the bike on the back wheel make it go completely?

Sounds like you have an air bubble in the system . You need to remove the calliper and tap the cable quite hard a few times when bleeding . Should help .

The caliper was taken off when it was bled, also I did tap the caliper and hose quite a bit, pushed quite a bit of fluid through.

Did you pump the lever with the funnel attached & wind out the free stroke adjuster?

I do don’t wind out the free stroke adjuster but pumped the lever. I follower the shimanso bleed procedure pretty much to the letter. Ill try again tonight.

Try winding it out a few turns, then pump the lever with the funnel attached, as it can quite often trap some air.
Failing that, I’m out of ideas as they’re usually really easy & straight forward to bleed.
It could be that one of the skirt seals has failed in the master cylinder, in which case they’re not officially repairable, so it’s new lever time (unless you’ve got a spare lever to scavenge the bits from)

This will fix it!

http://www.epicbleedsolutions.com/blog/5-minute-shimano-mini-bleed/

I’ve recently had a similar problem with an XT.
First off, I’ve been a big fan of M785 XT. I have them on three bikes. Yes I know how to bleed them and yes I know about the free stroke screw.

However, I’m now experiencing problems on a rear brake that is about 3 yrs old.
I can no longer bleed it bottom-up. It seems the master cylinder has a blockage which is isolating it from its reservoir and not allowing fluid to flow in either direction, be that bottom-up when bleeding or top-down where it’s no longer adjusting as the pads become worn.
Breaking the hose joint at the lever allows fluid to flow bottom-up, so it’s definitely a lever end problem.
Anyone experienced similar?

The levers are easy enough to strip down and clean. Master cylinder, take off cover and screws and clean. Only one small grub screw holds the lever together, the rest just pulls apart. There’s a tiny rubber bung covering the grub screw (which I think is 2mm hex) about 10mm from the back of the reach adjust screw. Get the bung out with a pick, remove screw, then you can remove the pin that the lever hinges on. You’ll need to remove the bite adjust screw too. The rest of the innards just pull out. You’ll need to poke the piston with something blunt and it’ll spring back out enough to remove it, or use a small allen key an poke it in from the other side where the hose attaches. As long as the piston seals are ok and it moves freely in and out after cleaning, you’re golden. the piston should poke out of the hole about 5mm once you’ve put it back in. The seals tend to swell over time and cause the lever to stop returning properly. Mine are 2 1/2 years old and had that problem with one. New lever time, since spare seals aren’t available.

2 fiddly bits are: getting the lever that acts on the piston for bite adjust back in place can be a bit of a pain and the spring in the lever can be annoying too. Just make a note of the orientation of everything before you take it apart, so you know exactly how it goes back on.

cheers citizeninsane,
I’ve already had the bung/grubscrew/pivot/spring/lever out and adjuster backed right off as you describe, then tried to push the piston out but obviously wasn’t trying hard enough. Will have another go.

It’s just the piston and a spring in there. You can give it a good poke with a small allen key, it’s plastic so you won’t damage it.