1975 fuel gauge

[Tim666]

I have owned the truck for a couple of months now, fixing quite a few things from searching this site. My fuel gauge reads properly when I first start the truck but after about 15 minutes of driving it drops to empty and will not come back again until the next day. There has been lots of work dine to the truck before I got it. I did splice a wire coming from the sending unit that was just a taped joint, I removed the ground and cleaned it. I checked the wires to make sure it was touching exhaust but I think I am stumped.
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[Tim666]

I just discovered it is not a wiring issue, there is quite a vacuum inside my tank after driving. When I release pressure my gauge comes back. I have never owned a carbureted vehicle. There is a charcoal canister under the hood but nothing connects to it, there seems to be only 2 pipes on top of gas tank with the newer in tank pump.
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[Dead Parrot]

On my 75 heavy duty(no emission), two pipes on top. One is the gas to carb. On the second, there is a vent hose on top of the gas tanks. Each had a small plastic vent to keep water and bugs out of the tank. My gas caps are not vented. My guess is on yours, the plastic vents were replaced with a hose that ran to the charcoal canister. Since the canister is now not connected, check for a plug in the hoses that used to run from the tank to the canister.

[Tim666]

Thank you I will check that