1964 Fuel gauge stays at 1/2 tank

[Poppy Red]

I have a weird problem with the fuel gauge on the '64. it was reading erratic, so I bought and installed a new sending unit, made sure the ground was good. Checked and substituted a constant voltage regulator, still the same problem which is: start the car, gauge goes to 1/2 tank which is likely correct for the fuel in the tank. When I turn the car off, the gauge stays at 1/2. If I open the line isolator I installed, the gauge goes back to 0. Happens every time so somehow, there is a voltage retained to keep the gauge at 1/2 tank reading.
Any thoughts? I installed the isolator as I am lately having a leakage problem, small current, about .42 amps that drains the battery over two or three days. But i cannot determine if that somehow contributes to the gauge problem.

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[Ward 57]

I have a weird problem with the fuel gauge on the '64. it was reading erratic, so I bought and installed a new sending unit, made sure the ground was good. Checked and substituted a constant voltage regulator, still the same problem which is: start the car, gauge goes to 1/2 tank which is likely correct for the fuel in the tank. When I turn the car off, the gauge stays at 1/2. If I open the line isolator I installed, the gauge goes back to 0. Happens every time so somehow, there is a voltage retained to keep the gauge at 1/2 tank reading.
Any thoughts? I installed the isolator as I am lately having a leakage problem, small current, about .42 amps that drains the battery over two or three days. But i cannot determine if that somehow contributes to the gauge problem.

I'd start at the fuse block and pull one fuse at a time to try to isolate the circuit that has the drain. Your profile shows an '03 but a '64 I think still has an external mechanical regulator. Could be stuck points internally. Could be the clock but feedback to the gauge? Not sure how the isolator fits in but really shouldn't be needed.