Help please on solving this issue. I have M8000 and am aware of the many previous threads and issues with wandering bite point and 73 different ways to achieve a good bleed – but I think I’ve identified an issue and want to see of anyone’s had similar.

I have these brakes on a bike that doesn’t get used much, but when it gets used the brakes get heavily used to their heat limit on steep descents. I sometimes get some mild wandering bite point but can live with that…

What I’ve noticed is that I can bleed them, have tried a number of alternative methods, have spent ages chasing every last bubble out of the system, adjusting lever angles, tapping, swirling incense and playing inspirational mood music to try to achieve the ultimate bleed. I get a good bleed, lever feels good, am confident all the air pockets are out….but then out of interest after just a few rides I’ve gone back and attached the funnel and found there are loads of new large bubbles to be chased out of the system, and have done this now 3 times in a row. The brakes seems to operate alright as assume the air makes its way up into the reservoir, but have had issues previously after bikes been sideways in car boot and got to the trail with no brakes.

So I’m assuming that somehow the brakes must be sucking air into the system under use. I can’t see any leaks or anything obvious. How can this happen, could it be cracked pistons or faulty seals?
Interestingly seems to do it more in really cold weather- Is there a fix or should I point them towards the bin and get some Hope’s.

Apologies if this has been done 373 times already…

I should add that the obvious assumption would be that am not bleeding them right but the brakes have been bed professionally on two separate occasions by highly competent mechancics, and still eventually did the same thing. I’ve tackled them myself as am not too shabby a mechanic but lack the particular familiarity with this system, have always bled other models successfully and would usually only expect to bleed yearly, not after every few rides. Probably should have warrantied but now well beyond it.

Are the latest version of XT brakes still as problematic or have they solved the issues?

Wandering bite point can be caused by the discs. I’m guessing bite point closer to bars on occasion on long descents? Can be caused by different materials in shimano discs making the disc distort enough to push the pads back a bit. We are using solid discs and can solve that issue.

The air getting back in sounds like a leak though. We’ve seen that on occasion when the wee black o-ring on the bleed screw ends up on the funnel instead of where it should be with similar symptoms. Once they are bled they should stay bled unless something is wrong IME based on loads of hire bikes plus guide bikes with XT. Other than the issue above, once we had a damaged seal on the caliper, which sounded similar.

FWIW with the new 4 pistons we haven’t had any issues, we are using solid Galfer discs.

Sorry, I’ve just reread your post. You are saying that the reservoir fills with air but no issues with the brakes? I think that is fairly normal, certainly I’ve seen it before. I think it’s as the pads wear. It causes issues if the bikes are upside down, you need to pump the brakes a few times and then it’s back to normal? We see that a lot.

It’s more weird than that as the pads are barely worn, and there’s air introduced after just a few rides. I would normally assume that once all the air is chased out of the system that they would stay like that as a sealed system, yet somehow each time I check theres more air in there. Its possible I have a dodgy set as I think they’ve been the same from new, maybe bad seals in the master cylinder. I think its normally masked as the air travels up into the reservoir so the brake operates ok, but what I’ve found is that shouldering the bike or laying it on side and maybe accidentally pulling lever shifts the air into the line enough to have no brakes at all..which is irritating.

Must be a nightmare! I find that it’s just when they are vertical for a big, bumpy uplift that there is a problem. Then it’s like 20 pumps and it’s good to go!! Sounds like a dodgy set. Unless maybe someone knows better. The disc tip is worth a look though.

Thanks for trying Doug. Had a great day-out with some of your guides many many years ago as part of stag-do group, we just did the low level trails but had so much fun. One day we will return and do some proper rides with you, its such a fantastic area and a great company. Hope everyone is doing ok out there, roll-on 2021.

Have you pushed the pads all the way back into the caliper before removing the bleed screw? If not, then the drop in brake fluid level that’s normally taken up by the diaphragm, will instead be taken up by the air you’ve just let in.

Thanks! I’ve not done stag groups for a while, some were a lot of fun.

Is it happening to both brakes?

I had a similar issue with Saints and eventually I just bought a brand new lever. Problem went away immediately. I had long suspected a leak of one lever as there was a tiny bit of oil residue on the underside of the lever just below the piston. It was a tiny amount but even a small leak will be letting air in. It did my head in for ages. I tried every bleed method there was and nothing solved it.

If you have this on both brakes then maybe you’re just very unlucky or it’s something else.

Yes, both sides the same. Definitely something weird going on. No idea how to fix it…have my suspicion that their ability to let air in is related to the wandery bite point thing – I read somewhere the bite point thing may be cause by dodgy internal anodising affecting the brake lever piston movement, so maybe it lets air in at the same time…?

My experience was similar, fixed by a replacement caliper.

Small bubbles could be seen around the piston when pumped.

Right, shoot me down if this sounds daft, but could it be how tight the olives have been swaged at one of the cable ends when the brakes were first fitted?
I tend to tighten stuff up to a bawhair* before stripped.
I am not suggesting this,but you could try a ‘wee nip’ to see if that helps.

*not a true engineering measurement.  

🙂