I believe you are going to have to disconnect the plunger rod and handle. Also separate the gear case from the plunger. You probably have two or three location where this mechanism is bound up.
I'm thinking that once the rod has been raised during the pumping of gasoline there is something about pressing the handle shaft towards the gear case and that allows the plunger to go back down into the cylinder to start over. Sorry, It's been awhile since I restored mine.