Cleaning a sticky piston tonight, pressed the calipers back in and ‘pssst’ some oik shot out of what is apparently a ‘weep port’.

https://bicycles.stackexchange.com/questions/69356/what-and-where-is-the-weeping-port-on-shimano-mtb-brake-levers

What is this wizardry? Brakes seems fine though after a quick bleed at the lever. Or will i die tomorrow on my epic descent?

Is that new, pretty sure my 10 yr old XTRs don’t have that….

Sounds like the system was overfilled and you’ve damaged the diaphragm pushing the pistons home as there’s too much fluid for the reservoir to contain.

Used to mean a new lever needed, but Shimano have started supplying spare diaphragms for warranty use. Not sure if the UK have them yet though. Worth getting your LBS to ask Madison.

They’ll need to know model no and LH/RH as they’re asymmetric and different for servowave vs non-servowave levers.

Alternatively, find a pal with a dead lever (but with ok diaphragm) and use it as a donor for the diaphragm.

You can get a diaphragm for the rod levers as I needed a new one on my Ultegra lever a couple of weeks ago.

Thanks. I’m not in the uk however. I have an old spare xt lever so could just swap the whole thing, or borrow a deore one from the other bike.

It really does seem fine though… i’ve checked for seepage, nothing, brake works and feels solid. .

Yes, got a surprise when mine did that too. Mine did it a long time ago, I didn’t replace the diaphragm and it did not change the performance at all, so had forgotten about it. I remember looking into it and deciding the diaphragm is fairly thick where it would have to split to do this, came to the conclusion that maybe fluid gets around the edges before being ejected…I may be completely wrong in this regard so curious to know what the correct technical answer is, is it a feature? If I hadn’t witnessed the spurt I’d have never known it happened from the after-effects and feel no need to chase it. So maybe try them for a while and see what happens…

It’s a breather hole to allow the bladder to easily expand when fluid is hot. You’ll see it on plenty of brakes, some more obvious than others. Often just a little random looking pin-hole. Fluid can come out of there if fluid gets in between the outside of the bladder and the inside lever body / reservoir cover, or if the bladder splits, which seems to be really common with current Shimano brakes.

I’m assuming if the bladder/ diaphragm splits then they would continue seeping fluid past that end port, though I guess once the lever closes off the reservoir port that it’s not under all that much pressure, so maybe there’s more to it. Others in that link above seem to show a countinued leaking from that port afterwards and inability to hold fluid in the reservoir. Whereas mine showed no signs of leaking after she spurted…and appear to be happy.

As explained, it’s just a weep port if the system heats up and oil expands, or it’s overfilled and gets behind the diaphragm.

Had it a few times over the years, easy enough fix if you want to clean inside, but it does involve removing the lever blade, removing the little grub screw holding the diaphragm in and then putting it back together again, which can be painful!

As walleater said, if you watch some GMBN videos on YouTube about disk brakes they tell you all about it but yes to allow tiny bits of air to escape to keep the system working optimal. Said better than that by walleater

From experience, they appear to have more than one ‘weep port’.

Can anyone recommend a source for these diaphragms? I see them on ebay shipped from Taiwan but no idea whether there are good and bad ones.

I probably shouldn’t be able to put a hex key through here 🙁

Any bike shop can order them post free from Madison – when they’re in stock…. Out of stock just now, due back late August.

Googling the part no (Y1XK98030) should find something somewhere?

Thanks, I’ve taken a punt on one from Taiwan, hopefully it will be ok. Can’t find stock in the UK and SJS, usually a good place to look, list it as discontinued.

And for anyone wondering how this happens….just like the op I pushed the pistons back without undoing the bleed screw on the lever and attaching the bleed cup. User error 🙁

Not discontinued, just out of stock at the moment.

SJS seem to have them in stock.

shimano-xtr-blm9100-diaphragm-unit-y1xj98030

9100 and 9120 use different diaphragms

My cock up – I didn’t see the K/J difference in the numbers. Sorry