Jeff,
With the two filter arrangement (the proper one IMO) a valve on the inlet (tank side of primary filter) would keep fuel from siphoning all the way back to the tank if the fuel level in the tank is lower than the filter housing. And keep fuel from flowing out it tank level is higher.
With that valve and the Caterpillar manual primer pump on the secondary filter, changing filters is a breeze-- install both dry and use the manual primer pump to fill. No bleed screws to open, etc. Been doing that for over a decade. No primer pump-- get one, and until you do, fill the filters before installing.
