Did you try the online Sears parts site? There might be a breakdown there.

As for the spring, take a look at the plug that was removed, usually there is a rubber gasket disc on the end, take a look for a spring imprint.

It's one of those " I 'd have to see it " deals. If the small side is too small, the spring might flip to one side of the ball and keep it from seating. If the large side is against the ball, it might hit the bore before it contacts the ball.

The small ball lets fluid from the reservoir into the pump on the up stroke, the large ball lets fluid leave the pump and keeps it in the lift piston. Think of the way a mechanical fuel pump works.