[mfdemicco]

Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.

 

[cxwrench]

Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.

^This^ I ALWAYS use the old pads to push pistons back when replacing them. Excellent suggestion.

 

[Lombard]

^This^ I ALWAYS use the old pads to push pistons back when replacing them. Excellent suggestion.

This sounds like a given. I wouldn't think doing otherwise would make any sense especially with any sharp instruments.

 

[aclinjury]

Maybe you are better to push the pistons back with the brake pads in place; i.e., load over a wider area less likely to crack the piston.

from OP's description, the cracks occurred not at the end of the piston facing the pad, but at the end facing the oil, i.e, the end that's inside the caliper.

 

[mfdemicco]

from OP's description, the cracks occurred not at the end of the piston facing the pad, but at the end facing the oil, i.e, the end that's inside the caliper.

I was thinking in general, when dealing with those pistons, which are obviously brittle and fragile, that it would be a better practice to push them back in this manner. Anyway, maybe the piston got cocked when pushing them in and the one side bottomed in the bore and cracked the piston in this manner. I would think the piston could be pushed in more squarely, if they were pushed in by the pads. I don't know what happened; just postulating a cause.

Anyway, the manual says this

"Use a flat-shaped tool to push the pistons straight back in as far as they will go, while being careful not to twist the pistons.
Do not push the pistons with a sharp tool.
The pistons may be damaged."

 

[Seven1816]

Never used sharp tools, always used plastic tyre levers for pushing the pistons in.
I am going to take apart my other set of RS805, and amalgamate all the good pistons to get one set back up and running.
There is one glaring effect on cracked pistons is that the small shards will make their way to the seals and start to score and cut the rubber seal which would or could be detrimental.
My say is if your brake pistons are asymmetrical, and pushing back the stuck out piston feels tough and worse crunchy then cracked pistons are the most likely scenario.

 

[davesupra]

Never used sharp tools, always used plastic tyre levers for pushing the pistons in.
I am going to take apart my other set of RS805, and amalgamate all the good pistons to get one set back up and running.
There is one glaring effect on cracked pistons is that the small shards will make their way to the seals and start to score and cut the rubber seal which would or could be detrimental.
My say is if your brake pistons are asymmetrical, and pushing back the stuck out piston feels tough and worse crunchy then cracked pistons are the most likely scenario.

Any chance that the pistons were pushed back too far into the caliper at one time, when you were replacing pads? Otherwise, I can't see how/why the back side of those pistons would crack.

 

[cxwrench]

It would be nearly impossible to push them in 'too far'. I guess you could push 'too hard' and/or use the wrong pusher.

 

[nova_rider]

...I can't see how/why the back side of those pistons would crack.

Let me try to explain how/why based on my theory of installation misalignment as the root cause.

If the caliper is misaligned against the rotor, when the brake is applied, the piston pushes against the pad, but only a portion of the pad would contact the rotor initially, which puts an uneven load on the piston. Since there is a seal between the piston and the caliper cylinder, that seal can act as a fulcrum like a see-saw, pushing the "back" side of the piston wall directly opposite the uneven load against the caliper wall.

Granted we are talking about minute movements, but I suspect the piston wall is quite thin and not designed to take this type of bending load. Adding to the fact that it is made out of brittle ceramic, it is sufficient to cause the piston wall to fail. One way to confirm this is to examine the caliper body for any unusual wear marks on the cylinder wall.

 

[Seven1816]

Let me try to explain how/why based on my theory of installation misalignment as the root cause.

If the caliper is misaligned against the rotor, when the brake is applied, the piston pushes against the pad, but only a portion of the pad would contact the rotor initially, which puts an uneven load on the piston. Since there is a seal between the piston and the caliper cylinder, that seal can act as a fulcrum like a see-saw, pushing the "back" side of the piston wall directly opposite the uneven load against the caliper wall.

Granted we are talking about minute movements, but I suspect the piston wall is quite thin and not designed to take this type of bending load. Adding to the fact that it is made out of brittle ceramic, it is sufficient to cause the piston wall to fail. One way to confirm this is to examine the caliper body for any unusual wear marks on the cylinder wall.

I started to rebuild the calipers today, so I took out the seals, amazed the inside of the seals had lots of tiny shards of ceramic, feeling the piston area the reservoir behind the seal is actually a bigger diameter than the lip after the seal, so looking closely there is no way the piston can touch and scrape against the side of the fluid filled area. I can only deduce that Shimano are using or used some badly made ceramic pistons.