Dual tank questions

[45acpJr]

The PO installed some kind of mechanical valve under the driver side seat to switvh between tanks. I'm assuming one of the sender units failed or maybe it was just the switch. What is the best way to trouble shoot this? Also does anyone have a diagram of the fuel delivery and return hoses?

 

[Fixer]

The switch is probably a factory switch or one from a kit. That was the factory location on early trucks with dual tanks.

What is it doing or not doing?

 

[45acpJr]

In my search to understanding the dual tank set-up I came across these gems. They helped me so I figure someone else might make use of them