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Re
Unlikely but if caliper not designed for it, or pistons not all the way back due to...
hydrolock, too much fluid in master
cracked in half piston, the inside part of the piston which used to be one with the outside part, has broken company and is now jamming the whole thing, so you can't push them back (most white piston'd shimano's of the last 10 years).
New pistons wrong, rotor too thick or pad too thick or combination of all three
BrickMan-
It's a Hope E4 caliper with the new stainless pistons replacing the old phenolic ones. Plenty of clearance on the caliper, Hope says it's compatible with 2.3 rotors. Pads are pretty new standard sintered ones. I had it working ok before, just the new pistons are so smooth that balancing them is proving tricky using the Hope "hold the pads back with a screwdriver" method
TooTallTim
Does anyone here use a disc pad spacer when setting up calipers? I've recently started using 2.3mm rotors, and installed new pistons and the pad clearance is so tight that all my usual tricks aren't working. Just wondered if I should get one of those slide-on pad spacers or cobble something from an old aluminium drinks can