Avoiding hydraulic brake system overfill when bleeding with worn pads
The problem you are encountering is likely because the pressure during the bleed process forces the pistons out slightly - this means that in effect you overfill the system.

You are best to push the pistons back fully with the old pads in place, then remove the old pads and replace with a bleed block.

You can buy specially designed bleed blocks, or you can see how to make your own here:http://www.epicbleedsolutions.com/blog/how-to-make-a-bleed-block/