Master cylinder - brake pushrod

Pushrod is attached to the piston by a square "O" ring inside. Put the rod in a vise with the piston sticking out and I use two screwdrivers to pry the piston off the rod. It is held in place very tightly so so will need some force. You will need another "O" ring to re-install. See if your new M/C is the same set-up.