its probly just being stubborn. I dont believe than anything holds the pin in (except for the rubber ring)

Youll probly need a small nail set/hammer or something simillar to knock the pin out. once you get it started, then you can grab it with pliers and pull it out.

I dont see this in the exploded view, but check the inside of that stud to make sure there isnt a set/slug screw which presses against the drive pin. I know some 1/8 rigs are setup like that.