Many discussions about it leaking but no one has said what size we need.

Someone sells them on eBay for a few pounds

Leaking Shimano Calipers – possible solution

Scroll down this a bit and there’s measurements

It is 3mm internal diameter and 1.5mm width. The original ones are 3mm 1.3mm.

I ordered them and they are miles too small.My innacurate measuring suggests an internal diameter of between 6 and 7 mm.

I have swapped an o ring from an old broken caliper with success but don’t know what size it was.

I’ve just ordered these.
Will let you know if they fit.

If you have the old o ring and calipers you can measure it

https://www.theoringstore.com/store/index.php?main_page=page&id=41&chapter=6

6mm 0D x 1.5mm thickness.

Viton or Nitrile (also called Buna-n).

70 shore is the stock, which will work but the problem will reoccur. 90 shore is better.

I think it was Peter Verdone that first investigated this.

The shore 70 rubber is quite soft and after a while ‘takes a set’ of the interior shape of the transfer port. It doesn’t push back hard enough against the caliper and a micro leak ensues.

Harder shore 90 rubber pushes back more. The other alternative is an o ring that is slightly thicker, 1.6mm or 1.7mm. Don’t get 7mm O.D., they won’t quite fit.

I’ve successfully refurbished 6 M785 calipers, 4 of which are now on my kids bikes and working fine.

The same approach has also worked on a problematic Zee caliper, but they’re a different size transfer port seal and I can’t remember.

I’d say that fits.

Looks good. Do you have a side-by-side comparison of the new and old o-rings?

The old ones look slightly bigger. Could they expand over time,causing the leak?
Have rebuilt the brakes and given them a good squeeze with the bleed block in and no visible leakage.
I know my rear brake is ok so I’ll stick this one on the back and swap just in case.

I’d say that fits.

Sorry – I’ve just seen that you asked me about the size on another thread: Just as well I didn’t see it, as I would have given you the same size as above.

My Shimano o-ring is different to yours.

This is a Deore caliper – the o-ring fits into the recess by the red arrow and is indeed ~3mm ID x 1.4mm section I replaced it with one out of my Lidl / Aldi o-ring selection box – I *think* it would have been 3.1mm x 1.6mm (ID x section), but can’t be certain. (It worked, anyway.)

ETA:

The old ones look slightly bigger. Could they expand over time,causing the leak?

I suppose they could swell, but they could also just be bigger – they are made to various standards, but JIS is slightly different to ISO which is slightly different to SAE, etc.

Just sorted out my front brake. The old o ring was a baggy fit in the recess. There was also a black smudge next to the recess.
I now have two solid ,non squeaking brakes.
Let’s hope they stay that way, 18p well spent!

Really interesting thread, as well as the couple of others on here on this subject. Thanks to everyone who has contributed.

I have SLX 7120 that seem to have the micro leak going on as there are spots on the back of the pads around where the piston makes contact, so keen to tackle this job.

Is there any way of finding out the o ring size needed without opening up the caliper though? I only have one bike so would like to get the o rings before taking the calipers apart if possible.

Just realised how poorly worded my question was so let’s try again…

Are all Shimano MTB caliper o rings the same size or are there differences between XT, SLX, etc and 2 / 4 piston models?

Thanks

I don’t know about the difference between 2 and 4 pot.
It’s literally a 10 minute job to sit them . Just make sure you have a good torx driver as the bolts are pretty shallow.

Thanks @zippykona good tip on the torx driver.

I just split two leaky ones – a MT500 caliper and it had the smaller O-ring –  6mm OD x 3ish mm ID

The 4-pot MT520 caliper had a bigger one  – 10mm OD x 7ish ID.

Both off bikes 2-3 years old. So it looks like there are varying sizes.

Seconded what zippy said about a solid torx head – one of the bolts was stiff AF in each case and needed a vice.

Tbh, spots on the back of the pads is likely to be the piston seals, not the O ring micro leak.

Update.first ride this week brakes wailing and no power.
I’m assuming they are still leaking .Poo.

Assume you changed the pads?

<tick>

Carry on everyone !

Assume you changed the pads?

No, did put the heat gun on them, produced smoke and then had 20 miles of nice brakes.
Parked up for 5 days and even pushing bike in the shed and braking produces a squeak.
Is it feasible that oil contamination from the back of the pad has seeped to the braking surface?

IME contaminated pads will never be good again. Might be okay for a little till the clean surface wears off. But they’ll always start squeaking again.

Get some new pads in I reckon.

Is it feasible that oil contamination from the back of the pad has seeped to the braking surface?

This will absolutely be the case.

I will put some in right now…stay tuned.

New pads..plenty of power and no squeals..time will tell.

Great thread! 4 pot Deore (I think) calipers apart on the bench in the garage if sort of given up on. I’ll get my calipers out tomorrow. I have spare pads. Once buggered, they won’t be revived imo.

Bookmarked.