Gauge Rage

[HIGHWAY BY THE SEA]

Ever since I got the 68 truck of mine running I have had problems with the fuel gauge . After I got the truck running the gas tank had to be replaced because it was rusty . So I got a used gas tank and a brand new sending unit from LMC. My mechanic installed it in there for me since he was already rebuilding my carburetor and he said that it worked fine. But after I picked up the truck I discovered that the fuel gauge was not working so I checked the sending unit and found the float had sunk . LMC sent me a new float and when I installed it the gage still would not work . So LMC send me a new sending unit and I installed it today but when I install it it does not work either . With the key switch in the on position and the sending unit hooked up to power you can ground it by hand and it will work . But for some reason the sending unit only works if you hold it in your hand and ground it while it is hooked up . As long is the sending unit is installed in the gas tank the gage will not move unless you almost fill it up and then it only registers a quarter of a tank or so. Clearly there is some frustrating issue going on here but I cannot figure it out .