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I noticed a movement of braking disc when pressing rear brake and watching on rotor. It was moving about half millimeter. Is it normal for rotor to move minimally?

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Being Hydraulic it is almost certain the caliper is double sided piston, where the pad on both sides moves. (Single sided calipers have a piston on one side, and push the disk onto pad on the other side).

Presuming it is a double sided caliper, it is possible one piston is not moving, leaving the other piston to do all the movement and push one pad across to the other. Short term this is not a problem, but as the pads wear, the amount of movement needed increases, wears pads on one side more than the other. Eventually you cannot adjust the discs to have enough clearance when not breaking and enough lever movement to brake.

If you have a look, you should be able to see both pistons/pads moving. If one is not moving, the caliper needs a service. It could be as simple as removing the pads, and working the piston a few times. (Pushing the piston in (carefully) then lever to push it out (careful not to to push it out too far). Cleaning the piston walls with isoprop, then a drip of brake fluid to lubricate it will help.

Answer in this question might help, especially the Park tool video.

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Assuming you mean from side to side, many disc brake calipers push a piston in from the side and sandwich the disc between that piston and a fixed pad on the other side of the caliper. Part of the setup/adjustment process for such brakes is to set the fixed pad position as close as possible to the rotor without having it rub, so as to minimize the sideways deflection of the rotor when the brake is applied.

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  • Not relevant for hydraulic brakes as mentioned in the question title.
    – MaplePanda
    Jul 26 at 5:51

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