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

Yes, once you broke seal, but not when connected and pulling it up from suction tube in tanks; would just stay neutral when key is off.
I would guess you have a bad valve on your fuel return and a leaking check on your devco.
My truck fuel returned to the top of both tanks, so with any component weak it could have easily siphon out fuel.

 

When the intake hose was pulled off, I kept the Devco check valve in. Nothing was backflowing out of it.
I purchased a new one anyway. Going to replace it later tonight or tomorrow. Also, getting to the back of the head is easier than I thought it would be. Sitting inside the driver wheel well, I can reach behind the motor with 1 hand and hopefully take it off. It's a 12.7 DDEC4 60 series . 2003 Century Class

 

By your description you could also have an inlet restriction from filter to the tanks. Blockage not allowing enough fuel flow to the Davco. Does it run ok and maintain height in the bowl when running? Backyard check.... put the line to the davco in a bucket. Put a small amount of shop air in each tank and make sure you have a steady unrestricted stream of fuel coming out the line. If not trace it back to the source.

Also make sure the tank vents are clear. If not it can pull vacuum on the tanks and cause the same issue.

 

When the truck is running, absolutely ZERO problems and filter fuel stays level, no bubbles that I see. Also, no bubbles that I see in the fuel tank. Just drains back into tanks overnight. No wet spots due to dripping anywhere on ground.

Devco check valve seemed like it was holding fuel (I took off inlet hose). That vacuum must be something else for it to pull fuel up to the head and down back into tanks.

I'm probably going to replace all fuel lines next weekend. These are originals from 2003. Where are vents located on the fuel tanks? Up on top? I appreciate all the help !!

 

Ya can't tell you what you have on a truck with a Detroit, I have a Cummins, would be easy to tell where return valve is and where.
Now I have ran lots of 60 serires in shrimp boats, but #### that was 15 plus years ago.

 

Quick update.

After since the incident, I wiggled all the hoses and connections. Found no leaks. Tightened just a bit the connectors I could get at. Problem resolved itself.

The only thing I noticed is there is about a 1/2 inch more fuel level in the Davco in the mornings. Truck starts with no issues.

 

so what was the issue and how did you resolved it

 

I just went through this, it was a plastic ball valve in the davco filter housing it had some trash and the spring that hold pressure on the ball was getting weak the part was around 20.00 at freightliner.