As Murray said, you pulled the piston out of the m/c. The pushrod will come out of the piston if you pull it hard enough. There is a small o-ring on the pushrod that just pops into a recess iside the piston, The o-rings get hard and sometimes hold the pushrod in tight. Maybe don't bother with the old stuff... I'd buy a 4-hole to 2-hole m/c adapter, an adjustable pushrod, and a modern aluminum m/c from someone like Dr. Diff.
DoctorDiff Drivetrain Components