........Or flatten your tank in & bend sending unit / pick up tube.
My old '65, Mean Green, had the side auxiliary tank that was definitely not vented. It also didn't have a sending unit. My gas gauge for that tank was a 3 foot piece of hose I kept behind the seat. Cruising the backroads or around town it never built up enough vacuum to suck-in the sides of the tank. But going on long trips I'd top off both tanks and start the trip with the side tank. And when it was empty, if one was paying attention or didn't have the music too loud, one would hear the tank go "dong". If one heard that and reached down to switch the valve on the floor, all was well, but if one didn't hear it, the engine would sputter and then one knew the tank was empty. I miss having the extra tank, and it will be swapped in as truck gets its makeover.
Seeing these photos and reading the responses, I need to check out the cap on my current truck.
