I assume that you popped out the pistons, cleaned out any rust and then reinstalled the pistons. Did you put in new seals or did you use the old seals? You should have used new seals because that may be your leak.

Suggest you bleed the brakes all around again. Be sure to take the cap off the master cylinder and keep it full of fluid when you bleed. There should be no spongy feeling. Start with the rear, probably right - the caliper farthest away from the master cylinder.

Also I am not sure but your master cylinder may (like mine) have two compartments for fluid and I understand that it is hard to get fluid topped up in the rear compartment of the master cylinder. You might check.