1964 k10 gas guage problem

[cameyer]

I am new to this board and just recently purchased a 1964 k10 pickup as my first car. I am slowly going through it and getting everything working. When i first bought the truck i didn't know the gas gauge didn't work till i ran out of gas just 5 min away from the sellers house. I've been searching through this form and have followed everyone's advice on what to do to check it however have hit a wall and don't know where to go from here. so far the gas gauge only reads full with everything hooked up, I then disconnected the ground wire which i read should cause the gauge to read empty but the gauge still reads full, i think hooked the ground back up and disconnected the lead wire from the sending unit and the gauge still read full. the gauge is at empty when the truck is off but always reads full when running. someone please help me where do i go from here