Spongy Shimano XT M775 brake lever, but why?

My rear brake has been spongy for about a year now. It firms up nicely when i pump it a few times but then it goes spongy a while after if i've not used the brake. I've topped it up with oil (which looked like it didn't really need it), there do not appear to be any leaks anywhere (no bubbles in the oil), everything is done up tight, the hose isn't damaged. I'm not sure what to do…
I've had a few other pairs of Shimano brakes over the years and i've always happily bled and swapped them about with no trouble. They've always responded really well to bleeding and topping up. I'm going through the same process with my M775 rear but it doesn't seem to be helping. Any ideas?

Level the lever on the bars, zip tie it back and leave it overnight, that improves things but the problem does re-occur. They can get small air bubbles in the caliper, zip tie the lever shut and open and shut the bleed nut. Mine is the same, I just re-built a bike and totally flushed it through, still not perfect but a massive improvement.

Is it the servo kicking in?

Shandy,
Ok. Thanks. I'll give that a go.

Joxster,
I think so. The brake certainly seems to become sharper towards the end of the stroke. As i understand it, the servo wave is just a cam on the lever mechanism and the lever seems to be working just fine once it's been pumped a few times.

Or maybe the resivoir needs to be topped up a bit

Joxter,
No. It's as full of oil as it apparently can be. This is why i'm puzzled by the spongyness.

mine have been like this for a while now. I've tried bleeding them but it's made no difference. I'll take them to a shop when I get time, see what the problem is.

I had that with some Avids, front was sharp but the rear was shonky as ****. I think it was down to the cable route. I put on some Deore and gave the Avids to a friend who has been running them without any issues.

Exactly the same problem on my XT brakes. Strange and really annoying.

Just need a good bleed with clean oil.

Can be a slow process with that particular brake, but worthwhile!

Seems that those calipers can trap air in some devious places – give em a bleed then give the cailper a damn good thwacking with something firm but non destructive (the rubberised handle of a hammer works for me) while twisting em around into a few different positions, and then bleed again – bet you'll get some more air out.

I had a shake-down ride on the new Blue Pig, the only glitch was the back brake so I unclipped the caliper and let it hang underneath the lever with the bike on the stand. Jiggled it around a lot and finally got a couple of tiny bubbles out of it, now its as solid as the front. The caliper must hold air somewhere.

Just pushed the pistons back, bled and jiggled the caliper and hose. Will now go for a ride and report back later.

How was it then banjowhacker?!

Air in the calliper I reckon.

Tricky to get it out, I pulled the lever to close off the circuit at the top, then used a syringe to push and pull oil in and out of the calliper. Eventually got a few air bubbles out and it's been solid since. Got the idea from watching the avid bleed video on youtube.

With XT calipers you need to have them as low as possible when bleeding to avoid any trapped air. Bleeding the brake on a bike can mean the caliper is at an angle where bubbles can get stuck.

Thanks for this all – my front xt brake is awesome but the rear is as superspongy exactly as banjowhacker describes. Have ordered a bleed kit and will see if it improves!! Any ideas on why cable routing may be an issue as above??

Mine goes really spongy once the pad gets down to the last mm or so, I guess I'm probably running out of fluid in the resevoir. Could it be that the aftermarket pads have thinner backing so are allowing the pistons out further and drainign the resevoir?

Glenn, you basically want a vertical line from the caliper to the lever, if possible. Air escapes up the hose more easily that way, and if you still have a spongy lever you can usually narrow the problem down to the caliper.

Thisisnotaspoon, that happened to mine over the winter, I just threw fresh pads in because I knew I'd be pulling the brakes apart in the Spring. Now they are bled properly again, the old pads work fine.

Best thing to do is to force fluid through the system using a syringe from the caliper end. Air bubbles will be forced up through to the lever reservoir. The 'leave it overnight with the lever depresed and the reservoir open' trick is also a good recommendation.

Did this to mine and they are solid as a rock now. None of the traditional Shimano sponginess. Actually feel more positive like Avids.

Going to try the zip tie trick. have done it before with the reservioir closed and it worked for a little while. Ive also bled my caliper with the syringe – thats how I usually do it. Useful thread this one!!

You need the reservoir open. With it closed, the air doesn't escape.

You need to push liquid through the system using the syringe, not draw it out. Not sure if I made that clear.

Solarider – yeah, my post was a little unclear. Ive only ever bled them by pushing the fluid through the caliper using a syringe.

I'll give them a bleed and see what happens.

Incidentaly, if the brakes are setup to be bled the traditional way (top up resevoir, squeze lever, open nipple, close nipple, release lever, repeat) then some (but not all) designs dont like being bled the opposite way, you'd be better off sucking it through rather than pushing.

What solarider says. It's much easier and quicker from the caliper back up to the lever.

you lot need to buy some Hopes

On a Bluepig you have to take the calliper off and also unbolt the hose as far as the BB to ensure that there are no local high points on the hose. If you look at the Bluepig design with the Shimano calliper you can see that there can be a nice bit of air trapped in the calliper when the bike is level:

See how the rear of the calliper is much higher than the bleed nipple? Its an unfortunate result of the two designs (Ragley and Shimano).

http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/videos0/disc_brake_movies.html

http://www.youtube.com/watch?v=YXmWziLAlBU

Damn. Sorry guys. I completely forgot to reply… Good news! It would seem that the XT caliper has nooks for tiny air bubbles and requires careful bleeding. It's as solid as the front now. All working lovely 🙂

Shandy – thanks for that! My bleed kit arrives tomorrow so fingers crossed it'll work!

Interesting thread!

I have a new Ghost with XT brakes and they are spongy! Can I just confirm something about the technique with the lever closed overnight with a zip tie i presume, you actually remove the cap of the resevoir so you can see the oil sat there in it?
I don't have a bleed kit, but is this still worth trying?

Apologies if that's a stupid question!

Many thanks