I have been refurbishing a 1961 Chevy Fleetside pickup the last few months and I have been replacing gauges. The gas level gauge is new along with the sending unit. I get a 12 volt reading at both the sender and the gauge, but the gauge only registers 1/2 full every time the key is turned on. Then it stays there, no matter how much gas there is in the tank. Any one know what my problem is?