I’m running 2022 SRAM Red AXS. Recently replaced my pads (trying out Swisstop), and I cannot get them to stop dragging on the rotor. I get them mounted perfectly centered, but after the first brake pull either one or both of the pistons stays right up against the rotor.
The pads are new, so there’s very little space between the pads and the rotors. The Swisstops measure the same width as stock SRAM pads, so that can be ruled out. I’ve had the bike for a few years now and haven’t had this issue in the past.
So far I have tried:
- re-bleeding the brake and removing some fluid to ensure it wasn’t over-pressurized
- re-centering the caliper (several different ways)
- cleaning and “massaging” the pistons, applying some DOT fluid to ensure the seals are lubricated, several times
- checking the rotor for obvious bends
- Braking power and lever pull is great front and rear, so I don’t think I’ve over-filled or under-filled the system.
- bending the brake spring to add preload (no effect)
I can force the pistons to fully retract, which leaves room for the rotor, but after the first pull they stay slightly advanced and contact the rotor.
What else can I try?
Thanks!
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Was the DOT fluid applied to the pistons in your attempt to lubricate the seals?
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Yes, that’s what I’ve seen recommended, so I gave it a shot.
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Does the brake lever return fully and freely? Does lever feel change once the pistons advance?
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Yes, braking is actually fine on both front and rear (only the front brake is having the piston issue). The lever snaps back when let go, and squeezing the brake feels normal.
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That’s not recommended. As a matter of fact, SRAM specifically tells you not to do it.
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Yep, I’m aware. I tried SRAM’s exact massage procedure a few times without any change. When it didn’t fix the issue, I looked elsewhere and found the recommendation about putting a little DOT fluid around the outside of the pistons.
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What method are you using to clean the pistons? The 3D brake blocks by Chris Heerschap really do a good job of being able to fully exercise just one piston at a time. This allows full exposure of the piston diameter for cleaning without risk of pushing the piston out too far. I’ve found that just holding back one of the pistons with a handheld tool while depressing the lever doesn’t really allow for full extension since I’m always afraid of pushing the piston all the way out by accident.
It could be that there’s something on the outside diameter that is causing excess friction that has not yet been exposed far enough for cleaning.
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I’ve seen those 3d printed blocks. I’m just holding one side in with a tire lever. I’m getting them far enough out that I would be kind of surprised if the last 1-2mm was causing the problem.
Is there a point where I should just get a rebuild kit and put new seals in?
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I’ve never overextended a piston, but could it be that when this happens, the seal could invert or otherwise unseat from the groove and cause the piston to bind? In any event, a rebuild kit should definitely fix the problem if you are comfortable doing that and the kit is available.
Good luck in getting to the bottom of this.
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If these are really 4 years old, may be time for a caliper rebuild w/ new pistons and seals.
There was a good visualization on instagram about how brakes need friction between the piston and seal to work properly. lubing the pistons inhibits this and can cause the pistons to not retract. The most common cause of sticky pistons is just gunk or corrosion between the piston and caliper body. Sometimes its impossible to get at the gunk without disassembling the caliper. At that point, it is better to replace the pistons and seals anyway.
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Ok. Rebuilding a brake is provably the only bike maintenance that I have not done, so this could be a fun project.
Early AXS 12sp calipers were a bit notorious for this, but usually it was only an issue when new. I’d start by cleaning the pistons again with alcohol and resetting them. Then with a bleed block installed, squeeze the lever once to apply firm pressure, and keep the lever there with a strap. Leave it for an hour or more.
This was the procedure recommended by SRAM when I had this issue with the new ones. I imagine the pressure over an extended period of time would ensure any assembly grease would be fully squeezed out of the square seals and help it grip the piston.
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thanks for that.
would it make sense to disassemble the brake and clean it out completely, so I know there’s no debris or incorrect grease or twisted seals?
Yes !
At this point I would disassemble the caliper(s) completely and thoroughly clean them. Those are 2 piece calipers, right?
Install with new seals amd NEVER EVER lube the pistons as lubing them does provoke exactly the kind of behavior you want to get rid of.
To follow up on this thread if anyone reads it in the future - the advice was right, the pistons weren’t retracting due to my putting DOT grease on them. I rebuilt the caliper using only DOT fluid as indicated in the SRAM service manual and they work perfectly now.
Also, I could not find a SRAM Red piston/seal kit in stock, so I bought a Force kit. Everything was identical except for the bolts - the new seals worked just fine.
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