Fuel Gauge Reading

[vetman]

got delivery of the 98 with 1/2 tank. Driving and gauge steadily going down. When the red light came on, I filled the tank with gas. The gauge never indicated a full tank but read about 1/8 and after 30 miles, red light came back on. I know the gas went into the tank.
With the Durametric, I can get the gauge to read 1/2 and full, so I guess its the sender? Why did it work until I filled the tank. I did swervy thing like warming the tires to see if I could shake it loose, but no.