fuel swap
 

My 87 K2500 with dual tanks will push the right fuel into the left tank when it is parked and not running, until the tank is try. Any one have any ideas why this is happening? I don't know if the left tank is sucking the fuel into it or the right tank is pushing it to the left. I don't see how it can do this as the internal valve shouldn't let it do so and I can hear it switch when I activate the rocker switch in the dash.
I want to check my fuel lines that go to the transfer valve under the truck, does any one have a diagram of the hose locations on the valve so I can check this? Thanks