1971 C-10 gas gauge ot working...

[68Gold/white]

I helped a friend locate this 71 C10, around Covid time, March 2022.
It seemed like the gas gauge worked when he got it, but has seldom work for quite a while.
I had the sender out of it, recently. This is a California emissions pickup (350), sender has 3/8 and 1/4" pipes on it.
I tested the float movement with an ohm meter. That functioned fine.
Right after I reinstalled the sender it went to 1/8 tank reading, which was likely correct. Now it just reads a fuzz off the "E" mark. I put $35.00 gas in it a minute ago, gauge did not move....

I'm wondering what the common problem could be with these pickups, if there is one...
Gauge cluster is original, old...it is the 6 hole type.

Thanks!!!