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Take the pads out and squeeze the levers a few times, don't let the pistons pop out, but enough you can see about 5mm of the sides of them, you may need to hold the non sticky one in to encourage the other one. Clean it with isopropyl on a cotton bud on the sides, then apply a little mineral oil and push them in and out a few times, push them in and give everything a quick clean with isopropyl and try setting it up again.
snottyotter
@Ben689908
I think I have what's known as a sticky piston on my front GRX disc caliper.
On pressing the brake lever, the pistons do their thing and push the pads against the rotor, but then one of the pads doesn't retract all the way back... so rubbing.
I took everything apart, made sure both pistons were pressed fully home, reinstalled the pads and centred the caliper by eye before tightening the mounts. Pressed the brake lever to apply the pads against the disc, and released. Same piston/pad stays further out/over... so rubbing.
This is a sticky piston, right? And I can try to resolve it using brake cleaner? I am presuming internal seals are fine - this bike has very few miles on it.
(at the moment, I have the caliper centred allowing for the sticky piston but there is so little room that it took me ages and any slight deformity (talking half a mm) happening to the rotor will result in rubbing again - so I'd like to sort)