When I put a new universal sending unit in my '38 I had an issue with the rheostat having so much friction that the float wouldn't raise the arm. It also was so light it wouldn't fall all the way to the bottom. I made my own float arm out of a clothes hanger with a longer float leg and put two floats on it. It rose to the top OK but wouldn't fall. I then put a small lead fishing weight on the arm and it worked normally. Several months later I found a good used OEM sender. When I pulled the universal sender out it had raised & lowered enough to loosen it up to the point it would work normally. You may be having the same issue. Pull the sender and immerse it in a container of gas & watch the float to see if it raises all the way to the top.

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