77 c10 gas tank problems

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77 bigten

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So the other day I filled my main tank on my truck. Drive to work and when i am ready to leave i'm at half a tank. Switch to my aux tank and it is now half full. My uncle had the truck for 4 months and swapped the original pump with a pump from an 84' which has a return line that has been capped at the pump. What would cause this an how can I fix this?
 
You dont mention which engine you have but isnt the correct pump available? Has this problem been occuring since the pump switch or it is something new? If you are stating the gas station pump shuts off too early than you have a venting problem. If you are stating you really do have a full tank and it reads 1/2 than you probably have a tank sender issue.
 
I have the 350 5.7L. The pump is not the problem. I am trying to figure out why my main tank when filled, drained into my aux tank. I filled it, drove to work, parked it for about 4 hours. Went out on break and it had drained in to my aux tank. Aux tank was empty an when i drove home both tanks were at a half tank.
 
it sounds like the control switch is permitting gas to flow from one tank to the other. Pressure in the main tank would cause the gas to go to the aux tank assuming the aux tank vent is functioning to let air out. This link has a wiring diagram for the switch.

http://67-72chevytrucks.com/vboard//showthread.php?t=535702
 
I have checked the valve, the vent, and the lines. It has not done it since the last time. If the filter that is inside the quadrajet is clogged could that cause the lines to over pressurize and cause the problem i had?
 
You would have driveability issues if the filter was clogged.
 
Found out that the problem is the fuel tank selector valve but can not find and original replacement. All I can find is an aftermarket POS with 6 ports and bad reviews that i would have to rewire the old wires to for it to work. Anyone know where i can get and original replacement part????:confused:
 
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