Unfortunately there are only two options.rebuild the caliper with new seals($~40Cdn)or replace with a rebuilt($~60Cdn).Rust inside the caliper causes the seizure.Brake fluid absorbs moisture and holds it (for a while)then it makes it's way to the lowest point in the brake system(the calipers or wheel cylinders).This is why it's critical to flush the brake fluid once a year.If you're on a tight budget and need a temporary fix try to seat the piston back into the caliper with a C-clamp.Then fit new pads($~30Cdn)and you should get a few thousand km before you have to really fix it.Good luck
