Shimano RS785 caliper issue

[bigbill]

My gravel bike has 685 levers and 785 calipers. Garro built the bike and I installed the components over three years ago. I've had zero issues but noticed the brakes levers feel spongy so I decided to bleed the brakes. The front was super easy, removed the pads, installed the block, and used the Shimano kit. Got some bubbles and good to go. The rear, I pulled the pads, noted I need some new ones soon, and couldn't put the block in due to one piston being extended. It looks corroded. I put the pads back in and bled the brakes with the travel spacer installed. They work and make no noise.

I guess I could bleed the caliper and try to push it back, but is it worth it? A new caliper is around $60.

[steve garro]

My gravel bike has 685 levers and 785 calipers. Garro built the bike and I installed the components over three years ago. I've had zero issues but noticed the brakes levers feel spongy so I decided to bleed the brakes. The front was super easy, removed the pads, installed the block, and used the Shimano kit. Got some bubbles and good to go. The rear, I pulled the pads, noted I need some new ones soon, and couldn't put the block in due to one piston being extended. It looks corroded. I put the pads back in and bled the brakes with the travel spacer installed. They work and make no noise.

I guess I could bleed the caliper and try to push it back, but is it worth it? A new caliper is around $60.

Take the pads back out and gently try to push the piston back with the O end of right around a 10mm wrench

- Garro.

[giordana93]

If I recall, the Shimano recommendation is to clean the (extended) piston with isopropyl then put a drop of the brake fluid/mineral oil on it before pushing it back in. A tire lever is also a good way to get them pushed back in. Obviously you will want to wipe it all down again with alcohol to remove any residue of the oil before installing pads.

[darkmother]

I had the same issue with my XT rear caliper-the seals get stuck to the bore. Be careful with the pistons, they are ceramic and quite brittle. I ended up cracking one of them trying to push it back into the caliper.

An SLX caliper is a bit cheaper, and functionally identical.

[Tristan]

Take the pads out and put mineral oil over the pistons. Cycle the brake a few times being careful that the pistons don't extend too far.

Give everything a really good clean with a dry rag.

Put the pads back in - they'll protect the caliper - then twist a clean screwdriver between them to push the pistons back.

You should now be able to place the bleed block. An old set of pads is good to keep around for the job but typically I only do this when I'm replacing pads so it's always the old ones anyway.

[bigbill]

Thanks for the good advice. I have some new pads arriving tomorrow, I'll try the different solutions to free up the caliper. I still have air in my hydraulics. A simple bleed didn't really solve anything but I can pump the brakes and get them to work so it's just a matter of bleeding more and perhaps pushing some fluid up from the caliper.

[dashDustin]

i've got little to no experience with these calipers. are the pistons ceramic?

[j44ke]

i've got little to no experience with these calipers. are the pistons ceramic?

Yes, they are ceramic. https://www.bikeradar.com/news/shima...liper-failure/

There are replacement metal-backed (or so it looks) pistons for 785 brakes on Amazon. No idea if this is cause and effect though.

[dashDustin]

Interesting. We have had some piston failures, but none resulted in fluid leak. They all felt like....

Bad bleed jobs with stuck pistons. No oil was actually getting through the line, but it took doing a full system bleed to figure it out. Couldn’t push oil up from the caliper to the MC. Or oil could get pushed up, but took more effort than it should. Each time the back side of the piston splintered.

[bigbill]

Interesting. We have had some piston failures, but none resulted in fluid leak. They all felt like....

Bad bleed jobs with stuck pistons. No oil was actually getting through the line, but it took doing a full system bleed to figure it out. Couldn’t push oil up from the caliper to the MC. Or oil could get pushed up, but took more effort than it should. Each time the back side of the piston splintered.

The stuck piston unstuck during the ride, so that put the rear brakes out of whack. I was descending by shifting my weight back and working the front brake. Didn't die. The rear pads are almost gone, I'm not going to bleed them again until I have new pads later this week.

[bigbill]

Tonight I got the pistons unstuck with the rubberized handle of a Park cone wrench. I was able to bleed the brakes followed by new pads. Good to go.

[steve garro]

Tonight I got the pistons unstuck with the rubberized handle of a Park cone wrench. I was able to bleed the brakes followed by new pads. Good to go.

Man.......the dust in AZ had been just evil the last # of months

- Garro.

[bigbill]

Man.......the dust in AZ had been just evil the last # of months

- Garro.

It has, but the rain this week has been nice. Today is damp and cool, but tonight through Tuesday is pretty much constant snow. But not Flag snow.