Davco filter (clear) housing empty overnight. No FUEL !

Wheres your fuel return to? Do you have a line to tank with a back pressure valve off fuel system, could be it has a bad sealing surface.
Check on your devco, is it vertical or horizontal, ball could be slightly off seat; or the seat could be out of round.
Plug the hole on inlet side of devco after you shut down for the night and see if problem still exist. Put a ball valve in for ease; then it could just stay in system. Brass what 5/6 bucks at hardware store.

 

Filter vertical yes, how about check valve. Even a spring loaded ball that sits horizontally can seep down overnight if components are out of round; even the dart types can wear on one side.
Yes ball valve inlet side of filter, good isolation point, while your isolating what the cause is. Remember to open in the morning
Process of elimination now. Slight injector leak, fuel system check, devco,etc...just a little air and everything high runs south to tanks

 

Weak spring will not keep ball tight on sealing surface, filter housing will siphon back. Weak spring in return line relief valve to top of tanks would make one big loop. All you need to drain fluid to the lowest point is a vacuum break somewhere.
all depends on the type of set up you have.
Fill a clear hose up with water, put one end in a bucket of water, raise the other end above you head. With your tumb over end above your head, hose will not drain, give it a little air and out the fluid goes. This is how your diesel is draining back. #### runs down hill.

 

The spring is not supposed to be strong ,suction pulls the ball off the seat. Take off the inlet hose and fill the bowl to the top with the cap off. If it leaks out the inlet the ball seat is bad and fuel is draining back to the tank. The only other option would be an external leak.

 

Yes, correct.