With this type of caliper, with an integral emergency brake, the piston has to be rotated back into its bore while constant pressure is applied. There is one proper way of doing this, and a few ways that might get you out of the woods.
The proper way is with a caliper piston retractor tool which looks like this:
This picture should give you a good idea of what I'm speaking of:
It's quite possible that the chain parts stores have these for rent, but I'm not sure.
Now, the more complicated methods (all will require a lot of inward force as well as a turning motion):
A set of needle nose pliers, with each tip in the caliper piston notches.
You can try one of these on the end of a 3/8" extension:
I've found that if I use anything other than the proper compressor tool, it take a LONG time to get them rotated and pushed back into place.