Sending unit float?

So finally pulled the sending unit from the tank and tested it. whlile manually moving the float up and down the guage works. I put the float in a bucket of water and it floats, when i reinstalled everything it is still showing empty.

I presume the problem is i need a new float, my question is can I put replace float with a cork? or is that bad? none of the local autoparts stores carry it. Has anyone tried that, I think it will work, let me konw your thoughts.