All Classified forums require a Site Supporter subscription. It is only $25 per year and you can buy and sell whatever you like on the forum. To become a Site Supporter, go to Your Profile and select "Paid Subscriptions."
You probably have the lines plumbed in incorrectly. 1/4" Teed vent, 5/16 return, 3/8" feed. Check the canister hose routing as well. Refer to the diagrams in the technical forum.
I just went through this with a 85' with dual tanks. The valve was either not switching all the way or leaking inside. The truck literally pumped gas from the driver side to the passenger side and ran it over to the tune of 2-3 gallons. As if this weren't dangerous enough, it also filled up the vent lines to the canister until it was flooding the carb.It gave me a pretty good scare when all that fuel kept flowing from the passenger tank. A new valve cured the problem. I have driven it to work for the last two days and it is back to normal.here is the post:http://forum.73-87chevytrucks.com/smforum/index.php?topic=37300.0good luck