You should be able to unhook the wire from the sending unit and the gauge should read empty, and ground the wire and the gauge should read full. If not, something is wrong between the sending unit wire and the gauge.
It should be the same as my 1950 coronet. The brake light pressure switch goes to the turn signal switch first, then to rear brake lights. There are six wires coming out of the turn signal switch by the steering column, on 2 3way bullet connectors. one of them is the brake signal in. You can check there for brake signal. I made a 3way pig tail of bullet connectors and unplugged the turn side wires and wired the brake signal directly to the taillight wires under the dash. It was in the same 3way connector. this way you can test your circuit from the brake switch to the brake lights. I had to buy a NOS aftermarket turn switch on Ebay and take it apart and file the contacts flat to make it work right.
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Harley PHD's post in 1949 Plymouth fuel sending unit was marked as the answer