No run 300d 2.5 turbo mystery
 

My 300d will not start if it sits for 48 hours. My indy says that there is a check valve in the IP that prevents fuel leaking back that may not be working and thus permits a drain to the tank,and/or I have a small air leak in the fuel return line that permits air to displace fuel in the pump.

The car runs well as long as I use it every day, or at least start it and let it run for a few minutes....but let it sit two days, forget it. I have to try to start it about 10x then: it starts, runs at idle for 6-7 seconds, stops. An hour later, it starts, runs 10 seconds. I go through this routine which is a PITA as you can figure...

Suspect your indie is on the right track looking for a fuel leak.

Try keeping the tank +1/2 full and parking with the nose pointed downhill or set the rear wheels up on a 2 x 4 and see if that helps.

There is a Over Flow Valve (pressure control valve; controls the fuel pressure and fuel return from the IP) on ther rear top of the IP near the idle adjusting screw that has a hose going to the fuel return on the fuel filter. It is not there to stop fuel bleed back.
However, the fuel supply pump on the side of the IP has at least 2 one way valves.
As stated before it sounds like an air leak any where from the primer pump back (if you have one).
After that it is possible that the supply pump valves are not seating due deposits as the mesh on the little plastic filter is not that fine and over time stuff can build up.