1. Did you open the bleed screw when you pushed the pistons back in? If you force fluid back toward the master cylinder sometimes crud from the calipers or hoses will block the fluid return holes in the master cylinder and cause dragging brakes. It is especially important on cars with ABS that fluid not be forced back upstream from the calipers, because there are lots of expensive parts with ruinable seals between the master cylinder and the calipers.

2. Sometimes, when a car ages, the inside of the flexible brake lines begins to deteriorate before the outside, and chunks of rubber can form blockages which cause drag, especially, again, if the caliper pistons were forced back without opening the bleed screws.