M785 rear calliper - where is the leak?

c3024446

Likes Bikes and Dirt
My rear brake keeps developing a bubble in the calliper, and I don’t know what’s causing it. On the bike it looks similar to this (i.e. bleed screw facing down):



I’ve recently put a new shimano BH90 hose on the brake (brakeset is 3yo, old hose was leaking contaminating the pads, note this was not a Shimano hose). After installing the new hose, I bled the brake correctly, lever was solid. Put the bike back into the horizontal position. After a couple of hours, I go to squeeze the lever, and it’s squishy.

If I stand the bike up vertical, the lever becomes solid again. With it still vertical - I connected a bleed hose to the calliper bleed nipple, squeeze the lever, loosen nipple, a bubble comes out with some the fluid. Re bled the brake. Test lever in horizontal after waiting an hour – squishy again! The process keeps repeating! A bubble seems to form in there, then move to the bleed port with the bike vertical making the brake solid, and floats behind the pistons when horizontal.

So there is a leak in the calliper somewhere. I have ziptied the lever to pressurise the brake, ridden the bike a lot, but I can’t find the leak! It’s like it’s a one way leak where the calliper is absorbing air under vacuum or something?

How can I find the leak? – It has to be at the hose join to the calliper, at the bleed nipple, or the piston seals. It can't be at the lever end. I’m sure it isn’t the infamous leaking banjo issue since it is a brand new hose. Is a new calliper the only solution?
 
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teK--

Eats Squid
3years old... I'd be banking on piston seals leak. Mine started leaking just under 2 years old.
 

c3024446

Likes Bikes and Dirt
Lunchtime bump before I buy a new calliper. Any way to fix this other than a new calliper?
 

moorey

call me Mia
Wait, I know that if I hold my bike vertical, levers go to bars, drop bike flat, all good. It's a weird thing, and I don't know why it happens, but it has no effect at all on the bleed or performance. Is there oil anywhere? Are you sure it's not just trapped air still in there somewhere that makes its way up?
 

teK--

Eats Squid
unfortunately no since you can't get standalone seals from Shimano. Someone on here posted a while ago that they were able to get one from a seals supplier though but there was no followup on whether they found the right one.

Do what i did, take the opportunity to upgrade :D
 

c3024446

Likes Bikes and Dirt
It’s definitely creating a bubble in there somehow. I’m thinking that after a couple of years the seals will allow air to easily enter the system at the calliper end, and this bubble floats up to sit behind the pistons. If you have a bike where the bleed port faces up at the rear like this, you wouldn’t have the problem until the bubble became really big:



My front brake has the bleed point pointing up – never had an issue with it.
 
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creaky

XMAS Plumper
I was getting a gradually softening rear brake with the M9000 xtr i've been running this year. There was some visible dust around the bleed port area even though it was done up tight. I swapped the bleed port for the little grub screw option that comes with the brakes. All good since then. Any visible dust collection around the caliper to indicate where the leak is ?
 

c3024446

Likes Bikes and Dirt
No luck seeing any dust. This isn't a gradual softening - it happens pretty much straight away.
 

richie_gt

Likes Bikes and Dirt
Wait, I know that if I hold my bike vertical, levers go to bars, drop bike flat, all good. It's a weird thing, and I don't know why it happens, but it has no effect at all on the bleed or performance.
This happens to me too, also after the bike has been upside down! I can only assume that the Shimano reservoir has enough space for air to be in the system above the lever (like a car master cylinder). If the bike is in any position but on the two wheels the air can go somewhere else in the system until it’s pushed back into the reservoir. I’m 100% sure I have no air entering/exiting since my last bleed, one pump of the brakes back to the bars then they are fine!
 

stirk

Burner
This happens to me too, also after the bike has been upside down! I can only assume that the Shimano reservoir has enough space for air to be in the system above the lever (like a car master cylinder). If the bike is in any position but on the two wheels the air can go somewhere else in the system until it’s pushed back into the reservoir. I’m 100% sure I have no air entering/exiting since my last bleed, one pump of the brakes back to the bars then they are fine!
This may not apply to all brakes but the diaphragm in my Shimano M505 gets rid of any 'space' in the reservoir and hence no air in the system. I fill up the reservoir so a little leaks out when I pop the diaphragm on. This is what I do and I can hang my bike any old way and the brake is always solid.

OP - Sounds like you have squeezed one bubble out with the calliper positioned vertically, have you done a full bleed with the calliper vertical?
 

c3024446

Likes Bikes and Dirt
I've bled it vertical, horizontal and other variations to try and remove any bubbles. Which makes for a solid bleed, then after a while, it turns squishy without me doing anything!
 

creaky

XMAS Plumper
I've bled it vertical, horizontal and other variations to try and remove any bubbles. Which makes for a solid bleed, then after a while, it turns squishy without me doing anything!
I think you'd be better off buying a full lever/caliper/hose assembly just in case the issue is with the lever rather than the caliper.
 

Flow-Rider

Burner
This may not apply to all brakes but the diaphragm in my Shimano M505 gets rid of any 'space' in the reservoir and hence no air in the system. I fill up the reservoir so a little leaks out when I pop the diaphragm on. This is what I do and I can hang my bike any old way and the brake is always solid.

OP - Sounds like you have squeezed one bubble out with the calliper positioned vertically, have you done a full bleed with the calliper vertical?
I would say you're onto something with the reservoir not being full to the brim, air will always travel to the highest point and get trapped. Until you rotate the component, it cant find a way out of the pressure side of the brakes. All of the Shimano seal leaks I have seen, have been an external visible leak.
 

c3024446

Likes Bikes and Dirt
I think you'd be better off buying a full lever/caliper/hose assembly just in case the issue is with the lever rather than the caliper.
I just bought a new hose, the lever is also kinda new as well (broke the first one), don't really want to buy a whole new brake. I also really want to identify the problem to help others out in the future.

The calliper will solve my problem. Surely.........
 

moorey

call me Mia
I would say you're onto something with the reservoir not being full to the brim, air will always travel to the highest point and get trapped. Until you rotate the component, it cant find a way out of the pressure side of the brakes. All of the Shimano seal leaks I have seen, have been an external visible leak.
Doesn't apply with 785's. You fill using syringe and funnel. It's a different system to the old bleed and fill reservoir system.
 

c3024446

Likes Bikes and Dirt
I'm somehow managing to get a bubble spontaneously form in the calliper after a couple of hours with the bike staying stationary and with the lever not being pulled. There HAS to be a fluid leak somewhere, I just can't find it. Cheers for everyone's help.
 

c3024446

Likes Bikes and Dirt
You could bleed the brake while it is off he bike...if you realy think this is the issue.
Interally run brake line, hence why when i bleed the brake the bike has to be vertical. When putting the bike down horizontal - gets squishy.
 

Flow-Rider

Burner
Doesn't apply with 785's. You fill using syringe and funnel. It's a different system to the old bleed and fill reservoir system.
Air bubbles would still be able to be trapped in the handle cavity. I would bleed them with the funnel, replace the bung back in and remove them from the bars, rotate in a few positions, recheck and see if fluid is up to the bung level, if not drip some in with a syringe. If he is hanging the bike up by the back wheel first or upside down, can almost guarantee stray air is travelling up from the brake handle into the calliper because it highest point.
 
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