It only pulls from the front tank. The supply line is only connected to the front tank. The return splits and goes into both. The hose connecting the 2 tanks levels the 2. It seems to almost empty the rear tank before the gauge starts lowering on the front. I kept the switch connected electronically to the senders so I can check the level in either tank, but the fuel doesn't go thru the switch any more to avoid the possibility of malfunction. My switch malfunctioned and was pulling from the front and returning to the rear with a big mess as a result. Being an old school trucker (33+ years) I remember the connecting hose between the tanks and adjusted for our setup.
Here is the link to the thread when I did the modifications: https://www.oilburners.net/threads/fuel-tank-switch-bypass.34465/
Here is the link to the thread when I did the modifications: https://www.oilburners.net/threads/fuel-tank-switch-bypass.34465/