Check out this link.
http://chevy.oldcarmanualproject.com/shop/1942_47/4247csm634.htm
It's a down-draft carb but the pumps are the same I would think. A new pump, (piston), should be in a rebuild kit. It's metal with a soft seal attatched to the disc at the end of the flat metal shaft, forming a piston.
Hope this helps.
Jacques


Union, Justice and Confidence