Brake education needed

My colleague at work ran into an unexpected result when he tried to push the piston into the caliper on the rear disc brakes. The c-clamp method that
worked great on the front calipers failed miserably on the back set.
My research on the Internet showed that the pistons on these type brakes must be rotated while they are being pushed in, and there is a special tool for the job.
My question is why do the pistons have to be rotated? What physical/mechanical feature makes this necessary?
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because the rear disk also acts as the parking/emergency brakes.
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OK, but what is happening mechanically when the piston is rotating that allows it to retract?

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"Harry McGuffin" > wrote

Harry, imagine that the back side of the piston is like a very large nut with coarse threads, and the adjusting shaft is like a very large bolt that threads into the piston. Basically, you are screwing the piston back onto this large bolt.
Ian
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you hit the nail on the head. it's mechanically retracting... when the parking break is activated, the lever rotates the piston mechanically like a screw (bypassing the hydrolics). the rear brakes uses both mechanical and hydrolic systems, therefore it is unwound to move it out for replacing the calipers/rotors. when finished, just use the hydrolics to push it back in (main brake pedal).
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