So I'm wondering what the post is for inside the bores. It's making it hard to get the pistons back in. Inside the pistons is a circlip holding something in. That something seems to grab the post inside the caliper.

closer

the something

That is a piston retraction mechanism. Along with the rubber seal, they pull the piston back to allow venting clearance between the pad and rotor. The rotor wobble pushes the pad back. That way the pads don't rub all the time resulting in unwanted friction.

Old post brought back. So I got two rear m-calipers from Parts Heaven. One had this set-up. The other does not have this set up, and is just your standard piston that looks like a cup. I need to send one back. So I can have two of the same. (I think) Which one. The one I have with the penis thingy is in not as good of shape.

for Mr Shea.

Or you could buy new ss pistons without the retractor mech.