2000 Silverado 2500, 6.0 fuel pressure issues and hard starting

[Mississippi Mike]

Sorry for this lengthy description but I have done everything I can think of. The truck has had a starting issue since I bought it a year ago. Otherwise, once started the engine runs smoothly even at idle. 

I installed a fuel pressure gage on the fuel rail test port. No fuel escaped while installing the gage. When the key is turned on, the fuel pressure goes to 55 psi then immediately drops to zero. Fuel pressure slowly increases while the engine is being cranked.  Cycling the key from off to on produces fuel pressure each time, but after the initial prime, the pressure quickly goes to zero. No fuel leaks have been found and I have traced both the fuel delivery and return lines from top to bottom. 

The previous owner had replaced the fuel pressure regulator, probably trying to correct this issue, but I ordered and installed a new Delco fuel regulator, vacuum hose and Shrader valve for the fuel rail port. There was no evidence of fuel when the vac. hose was removed with the engine running. With the engine running at idle, I have 50+ psi with the expected rise at higher rpm, but when you turn the key off, the fuel pressure still drops to zero immediately. No residual fuel pressure at all.

The next logical thing would be the fuel pump check-valve. Since replacing the fuel pump is a pain, I installed a high-quality in line fuel check valve by cutting the metal fuel line between the fuel pump outlet and fuel filter. The fuel filter appears to be fairly new. The fuel pressure drop remains the same. Next, I removed the fuel rails and fuel injectors looking for a leaky injector. I tested all eight injectors for resistance, and all produced around 12.2 to 12.7 ohms. Using a twelve-volt power supply and pigtails, I bench tested, energized and cleaned each injector with carb. cleaner and all sprayed correctly. I then filled each injector from the inlet with carb cleaner and found no leading injectors. Fuel injectors remained dry after reinstallation.

My truck fuel system includes a return line to the fuel tank. I guess my question is, how can the fuel system maintain fuel pressure without fuel being returned to the fuel tank automatically? Is there a check valve to close the return line when the engine is stopped? If so, I can't find it. Does the evap system control this? The computer is not throwing any codes. 

I must be overlooking something obvious, and I hope someone with knowledge of the GM fuel system will have an idea. Any assistance will be appreciated, and I thank you in advance.