These are the gaskets that have probably worn down. They are identical, but facing opposite directions. The bottom is for the presta valve, and the smaller hole wears down for it. The top gasket is for the schrader valve, and the larger diameter wears down on that one.

Switch the gaskets around while flipping them over, or just replace with new gaskets. As mentioned previously, these gaskets may be common among many different types of pump heads. Next push the gaskets into the pump head

Put the other pieces in the pump head ("reassembly is the reverse of disassembly"), making sure the rubber ball is captured between the other two pieces.