Its the job of the seals in the caliper to push the pistons back in after you release the brake lever. Due to the amount of heat disapated through the brake pads, pistons, fluid and caliper housing there is a good chance these seals have lost their flexability and thus ability to pull the piston back the fraction of a millimeter to keep the pads from dragging on the rotor. In the master cylinder the plunger needs to move to the rest position quick enough to release the residule line pressure back into the resevour. Also if the brake lever is not adjusted to allow full retraction of the the plunger this will also cause the pads to drag on the rotor.
