Many vehicles with rear disc brakes have the E-brake built in, meaning a pad change there must be more precise than screwing the pistons all the way in and throwing pads in. Most have a "friction pin" technology (not knowing your vehicle make or year doesn't help much). But what usually should happen here is the piston should be rotated and pressed inward only to the point where the caliper AND pads fit semi snug upon the rotor. Too much Pad-to-rotor gap will not provide adequite E-braking within the travel of the calipers lever.
I have seen instances where the E-brake would be applied and released several (sometimes many,many) times which gradually adjusts this mechanism within the caliper.