Shimano M785 brake, pads too close

[cribbit]

To my understanding on a hydraulic brake you're supposed to literally pry the pads apart (ok, push the pistons back into the caliper) then it sort of self adjusts from there.

The issue is I can only get the pads just barely far enough apart to get the rotor between them. It still rubs, on both sides.

What am I missing? Am I supposed to push the pistons harder? Losen a screw?

[NHAero]

Bleed a bit of fluid out.

To my understanding on a hydraulic brake you're supposed to literally pry the pads apart (ok, push the pistons back into the caliper) then it sort of self adjusts from there.

The issue is I can only get the pads just barely far enough apart to get the rotor between them. It still rubs, on both sides.

What am I missing? Am I supposed to push the pistons harder? Losen a screw?

[cribbit]

Bleed a bit of fluid out.

I've been one small step away from done on this build for like 2 months! Time to dive into bleeds I guess.

[dougefresh]

I've been one small step away from done on this build for like 2 months! Time to dive into bleeds I guess.

A full bleed should not be necessary. Clean a screwdriver really well with isopropyl alcohol, open the bleed screw on the lever and wedge the screwdriver between the pads pushing the pistons back in. Close the bleed screw. Clean everything well again with the alcohol, reinstall the wheel and give the lever a few squeezes.

[joosttx]

Have you pushed the pistons so that they a flush - flat?

[NHAero]

Thanks for adding that, I should have been clearer in my response. Totally agree!

A full bleed should not be necessary. Clean a screwdriver really well with isopropyl alcohol, open the bleed screw on the lever and wedge the screwdriver between the pads pushing the pistons back in. Close the bleed screw. Clean everything well again with the alcohol, reinstall the wheel and give the lever a few squeezes.

[cribbit]

Tried that. Pads still rub and now the brake lever has to go all the way to the bar engage. I probably did it wrong. Shouldn't do things late and tired.

[oldpotatoe]

Tried that. Pads still rub and now the brake lever has to go all the way to the bar engage. I probably did it wrong. Shouldn't do things late and tired.

Can you move the pistons in at all? Perhaps they are 'stuck' in the caliper.
Start from scratch..they obviously need a complete bleed but when open, system 'open', move the pistons into the caliper. Lots of vids, etc on freeing 'stuck' pistons in calipers.

[Jaybee]

Can you move the pistons in at all? Perhaps they are 'stuck' in the caliper.
Start from scratch..they obviously need a complete bleed but when open, system 'open', move the pistons into the caliper. Lots of vids, etc on freeing 'stuck' pistons in calipers.

Yup. I think YouTube is your friend here. Also, MTBR and Pinkbike forums have specific brake and wrenching forums with users who have no doubt faced this problem before

[Matthew]

Having a similar issue with my front caliper. Had my shop bleed my brakes this winter. Worked great. Rode a handful of times and started rubbing. I pulled the pads and appears one piston will not fully retract into the caliper body. Mechanic said my piston was cracked and causing my issue. New caliper for me. Could be your problem?

[dem]

Yes, needs more information.. are these parts all new? How much rubbing? Just noise, or would it actually drag down the wheel? Was it just rubbing in the stand after install, or did you actually go ride it?

Used/worn parts could be bad seals or a cracked piston as indicated - easiest would be to just replace, especially if you're about to go through the full-bleed dance anyways.

[AJosiahK]

I've never come across a situation when there is too much fluid. Assuming the mechs used all the right bits to bleed the brake then where is the actual problem ya know. but I would agree trying to at least let a little out could help, like juuuust a little.

i would also throw everything back on, and try and readjusting the caliper itself. easily done by loosening bolts slightly and holding the lever down, this should center the caliper a little better. Just tighten bolts down while holding the brake and hopfully that will lessen the amount of rubbing happening.

[NHAero]

I have, and on the same calipers, when putting new pads in. Didn't need to take much out, a tiny amount, but it made the difference.

I've never come across a situation when there is too much fluid. Assuming the mechs used all the right bits to bleed the brake then where is the actual problem ya know. but I would agree trying to at least let a little out could help, like juuuust a little.

i would also throw everything back on, and try and readjusting the caliper itself. easily done by loosening bolts slightly and holding the lever down, this should center the caliper a little better. Just tighten bolts down while holding the brake and hopfully that will lessen the amount of rubbing happening.

[Jaybee]

Having a similar issue with my front caliper. Had my shop bleed my brakes this winter. Worked great. Rode a handful of times and started rubbing. I pulled the pads and appears one piston will not fully retract into the caliper body. Mechanic said my piston was cracked and causing my issue. New caliper for me. Could be your problem?

Yes, needs more information.. are these parts all new? How much rubbing? Just noise, or would it actually drag down the wheel? Was it just rubbing in the stand after install, or did you actually go ride it?

Used/worn parts could be bad seals or a cracked piston as indicated - easiest would be to just replace, especially if you're about to go through the full-bleed dance anyways.

The cracked caliper or seal seems like a good guess. I've never encountered any real challenge pushing an intact piston/seal assembly back to where it needed to be.

[Matthew]

I tried using a plastic tire lever to ease it back in place to no avail. I compared to the rear and it was pretty clear both pistons in back retracted all the way so figured something was impeding the travel of the piston. My mechanic said there were tiny bits of piston material in the caliper body causing my problem. Didn't know this could happen. Not terribly familiar with disc brake tech. I do know in just the handful of times I've monkeyed with rotors I've lost portions of fingernails by me being an idiot.

[dddd]

I tried using a plastic tire lever to ease it back in place to no avail. I compared to the rear and it was pretty clear both pistons in back retracted all the way so figured something was impeding the travel of the piston. My mechanic said there were tiny bits of piston material in the caliper body causing my problem. Didn't know this could happen. Not terribly familiar with disc brake tech. I do know in just the handful of times I've monkeyed with rotors I've lost portions of fingernails by me being an idiot.

I've come across poorly-retracting hydraulic calipers a couple of times, I found that dousing the piston area in situ with rubbing alcohol, before again forcibly retracting the pads, improved the retraction noticeably.
Especially in cases where the bike has been sitting, I believe that silt and corrosion can impede piston motion, while the alcohol helps mobilize the crusty stuff that is restricting free movement of piston and seals.

[Matthew]

I doused the piston too and tried cleaning etc. Still could not move it. Definitely something askew so I am having the shop replace it. I bought them used and when they were bled the mechanic mentioned the fluid was about the dirtiest he's ever seen so not terribly surprised it was malfunctioning later on.

[cribbit]

Had more time to look at it today.

https://i.imgur.com/SpEmYzn.png

Full size: https://i.imgur.com/T3u1wli.jpg

Both pistons can be pushed out but immediately come back to that position. When I take the bleed screw out I can see the fluid level rise.

Aren't these pistons supposed to go back to flush with the caliper?

[dem]

They will only retract enough to not rub, ie, self adjusting.. do you have the pad spring in there? Not obvious from the pic.

[cribbit]

They will only retract enough to not rub, ie, self adjusting.. do you have the pad spring in there? Not obvious from the pic.

Pad spring is in there.

[cribbit]

Any other ideas? This is the last thing I need for this build.

[dddd]

With the seals doused with rubbing alcohol, how many cycles of forced retraction and squeezing onto the disc rotor did you cycle through?

When I've done this, I did it repeatedly, at least a half-dozen times before I checked for and noticed an improvement in the pad retraction.

This mineral fluid does not attract moisture so I would not suspect that it was behind the problem unless water somehow entered the system (which would be very unusual and difficult).

Double check the caliper alignment, could be tilted or twisted away from parallel with the rotor. Most important, note that the rotor does not tilt, twist or get pushed to one side when the brake is applied(!). Have the bike over a sun-lit sidewalk while testing this, the gaps and rotor movement will be much easier to see.

Note that many installations have the swivel washers present only under the bolt head, and with no space for the washers between the caliper and it's mounting post.
I've had to file the post faces a few times in order to correct this, since adding swivel washers would have pushed the caliper pads outward off of the disc's braking surface.

[NHAero]

There are swivel washers on the Avid BB7 calipers. I don't recall them on the Shimano 785 hydros I've installed, if we are referring to the rounded paired washers that allow for a fair bit of misalignment. I had one custom frame that was no trouble to set up with the road BB7s but produced rubbing with other calipers until I had the mount re-faced.

Have I been missing something?


With the seals doused with rubbing alcohol, how many cycles of forced retraction and squeezing onto the disc rotor did you cycle through?

When I've done this, I did it repeatedly, at least a half-dozen times before I checked for and noticed an improvement in the pad retraction.

This mineral fluid does not attract moisture so I would not suspect that it was behind the problem unless water somehow entered the system (which would be very unusual and difficult).

Double check the caliper alignment, could be tilted or twisted away from parallel with the rotor. Most important, note that the rotor does not tilt, twist or get pushed to one side when the brake is applied(!). Have the bike over a sun-lit sidewalk while testing this, the gaps and rotor movement will be much easier to see.

Note that many installations have the swivel washers present only under the bolt head, and with no space for the washers between the caliper and it's mounting post.
I've had to file the post faces a few times in order to correct this, since adding swivel washers would have pushed the caliper pads outward off of the disc's braking surface.

[bewheels]

This may have already been posted and I missed it but consider this to check whether the pistons are moving correctly.

Keep in mind that it is a closed system. So you have to manipulate the system in a way to force fluid where you want.

Remove the pads.
Use a sturdy plastic lever to push both pistons all the way in.
Take note as to whether one doesn’t want to go all the way in. Use alcohol and a q-tip to clean around the piston seal if needed.
Once they are both all the way in, hold one piston in place with the plastic lever and pump the brake lever to push a single piston out.
You will have to find a sweet spot with the plastic lever that allows you wedge it against part of the housing to hold the one piston down while having it not interfere with the piston that you want to push out. You have to hold that piston pretty hard to keep it from coming out. You will not be able to do this with just hand strength. Leverage is you friend as is a plastic lever. Do not do this with any lever that can damage the piston surface (screw driver or whatever).
Once you have the one piston pushed out a bit, push it back in.
Do the other piston.

Doing this will show you if anything is sticking.
Clean everything with that alcohol and q-tip.
Once you know everything is moving fine then you can eliminate that from the issues.

*Each piston may move out differently when compared to each other. This is normal.
**make sure you do not push the piston out so far that if comes out of the housing. They are deeper then you would think but don’t push your luck in pushing it out too far.

[vqdriver]

Had more time to look at it today.

https://i.imgur.com/SpEmYzn.png

Full size: https://i.imgur.com/T3u1wli.jpg

Both pistons can be pushed out but immediately come back to that position. When I take the bleed screw out I can see the fluid level rise.

Aren't these pistons supposed to go back to flush with the caliper?

it does this with the bleed screw out? so you can see the fluid rise but when the pistons return to that position the fluid level goes down again?

if i'm understanding that correctly, that can't happen due to hydraulic pressure with the bleed port open. sounds to me like the pistons are binding on something. time to drain the line, remove the pistons and inspect the pistons and gaskets.

tbh, i'd just drop it off at the lbs at this point.

[cribbit]

it does this with the bleed screw out? so you can see the fluid rise but when the pistons return to that position the fluid level goes down again?

if i'm understanding that correctly, that can't happen due to hydraulic pressure with the bleed port open. sounds to me like the pistons are binding on something. time to drain the line, remove the pistons and inspect the pistons and gaskets.

tbh, i'd just drop it off at the lbs at this point.

Yep, exactly. Bleed screw out -> push piston -> fluid comes up -> release piston -> fluid goes down -> piston stays where it was at the start.

Am I correct in thinking that the piston should stay where it got pushed in towards? That it shouldn't be able to push back out on its own?

[jtakeda]

Yep, exactly. Bleed screw out -> push piston -> fluid comes up -> release piston -> fluid goes down -> piston stays where it was at the start.

Am I correct in thinking that the piston should stay where it got pushed in towards? That it shouldn't be able to push back out on its own?

You are correct.

If you put the bleed funnel in your lever ( with a dash of fluid) and then push the piston in and then close the system does it still push in after you’ve closed the port?

Did you ever NOT use a bleed block when setting up the system?