How do you back up the piston in the caliper on the rear brakes of a 2002 X-type?

Answer

Sometimes if piston wont push back in try twisting it back in.

Its on a 'staircase' pawl. Twist. push, twist, push etc.

Use a 12mm (about 1/2inch) hex key to rotate the piston at center hole. no need to push, just rotate. Before rotation don't forget to let the handbake off, and brake fluid reservoir open.

Check the piston gaskets and replace if needed (Ford Mondeo MK3 compatible), the set for the Ford Mondeo is about 11$ per caliper.