Shimano uneven pistons / reset?

[PoorCyclist]

I have GRX 800 rear caliper and I can see the piston on the left side is sticking out more on the right side.

This came to my attention because the adjustment on the frame is about maxed out and the rotor almost touch the slot of the caliper's right side. It is currently clamping and retracting no rubbing.

I plan to push back the piston on the left side to retract it some maybe also press the lever a bit to extend the right side. Using the wedge part of the Hayes tool to pull towards the myself, hopefully will have even pressure (vs prying).

But I am nervous about all this because I heard the ceramic piston can crack while resetting the pads.
Can anyone explain how to best do this reset operation?

The chainstay mounting is milled (no paint) but don't know if it needs faced, I kind of want to reset the pistons for now.

Thanks

[Cemicar]

I've been pushing back those pistons numerous times, sometimes roughly but have had no issues.

Even Shimano itself causally says you can use something flat-shaped around you, and I normally use tire-levers because they're always available even in outdoors and resin-based so perhaps would not hurt pistons.

[Maddie]

Use a plastic tire lever for this. Pedro's work well

[kofsw4]

If you have a worn set of pads you can install those and push them apart using a flat screwdriver or anything really

[ms6073]

I thought I had read on a thread here that uneven piston actuation was most often either due to a sticky/bad seal or a micro air bubble on the effected side. If the issue is an air bubble, removing the caliper mounting bolts and tapping on the sides of the caliper while bleeding helps fix the issue.

[Cleaner]

This video is on a MTB but the procedure is the same for any hydraulic caliper. They use a specific tool (piston press) but as mentioned previously a plastic tire lever will work fine. Just be careful to not cock the pistons off axis when pressing them back in the bore.

https://www.youtube.com/watch?v=vQXFFgRButo

[PoorCyclist]

I went to a sports store and asked for the Park tool "press" in the video which Park say not to use on ceramic pistons. The person just told me to use a plastic tire lever (pads removed) I think I understand now, the white piston I'm seeing is the ceramic piston, if it cracks I think I would know.

[Karvalo]

I have GRX 800 rear caliper and I can see the piston on the left side is sticking out more on the right side.

This came to my attention because the adjustment on the frame is about maxed out and the rotor almost touch the slot of the caliper's right side. It is currently clamping and retracting no rubbing.

Just checking this - so the rotor is offset to the right side of the caliper, and the left piston has come out further to follow it? Is there actually a problem?

Does the left piston now have less clearance than the right? Does it contact the rotor before the right? If not, why reset it?

[PoorCyclist]

Everything still working for now but the rotor's right side is almost touching the caliper body's slot. The gap there is as small as the pad and rotor. Left side piston (the white round thing) is visibly protuding more than right side

[Cleaner]

I would suggest removing the pads and pressing both pistons into the caliper and then center the caliper on the rotor. Then observe the position of the rotor within the caliper slot, it should be centered in the opening. If it is not centered then follow the directions in this video.
https://www.youtube.com/watch?v=uk_nC9anQcM

If centering the caliper does not resolve the issue then look at the wheel. You may need to shim the rotor or adjust the dish on the wheel. If after all of this you think the frame is off then you will need to have it inspected using measurement fixtures.

[Karvalo]

Everything still working for now but the rotor's right side is almost touching the caliper body's slot. The gap there is as small as the pad and rotor. Left side piston (the white round thing) is visibly protuding more than right side

Again, so what if it's protruding more and that's all it's doing? Does it clear the rotor as much as the right piston does? Does it contact the rotor at the same time as the right piston does? In short - if your rotor is not centred in the caliper, why do you want the pads to be centred in the caliper?

Now, if I was you I would be looking to shim out the rotor so you can properly centre the caliper on it, but until then I wouldn't be bothering messing with the pistons uness you have an actual operational problem, because otherwise you might create one.

[Karvalo]

If centering the caliper does not resolve the issue then look at the wheel. You may need to shim the rotor or adjust the dish on the wheel.

Explain to me how redishing a wheel changes brake rotor position?

[grover]

If one piston is coming out further than the other it usually mean there is more friction between the seal and piston on that side. A 'sticky piston'.

Use a cotton bud dipped in brake fluid (mineral oil for Shimano) to clean the exposed pistons. Always do this before pushing a piston back. Then push the good piston back in. Hold it in and squeeze the lever to advance the sticky piston. Again, clean with brake fluid, then push the sticky piston back into the caliper. Then clean the caliper with isopropyl alcohol once the piston is back in so that no brake fluid gets on your pads.

The idea is that there is some contaminant on the sticky piston, probably introduced when the piston retracted previously, possible when it was pushed back in without being cleaned to install new pads. By cleaning with brake fluid you also lubricate it properly.

[kode54]

I thought the pistons were made from Teflon material rather than ceramic.

Like @Grover have said...usually dirt that makes the piston sticky or less smooth than the opposing piston. I use mineral oil to lubricate the pistons and actuate the pistons so I have more piston surface to lubricate, then press them in again. I do this a few times and test to see how much easier or smoother to press the pistons back in.

[Cleaner]

If centering the caliper does not resolve the issue then look at the wheel. You may need to shim the rotor or adjust the dish on the wheel.

Explain to me how redishing a wheel changes brake rotor position?

The centerline of the hub can be shifted relative to the centerline of the rim by manipulating the tension of the spokes. If you move the hub relative to this datum and the rotor is mounted on the hub then it will move as well. With centerlock rotors it is not practical to shim the rotor offset (on DT Swiss and Shimano hubs).