Fuel Regeneration Valve / Fuel Injector ?????

I have a 1998 ML320 that cranks for about ten + seconds before it starts. MB dealer said it had a leaking check valve in the fuel pump and replaced it along with the fuel filter. It did not solve the problem. I have learned that the fuel filter also incorporates the fuel pressure regulator. (It has four ports on it)
I found that if I turn on the ignition key for about 10 seconds before cranking the engine, it starts within a few seconds. When I turn on the key, I can hear a humming noise at the engine (which I assume is the fuel pump running) which does not stop even after running about 60 sec.
On the fuel return line, is what is called a fuel regeneration valve. (mounted on the fire wall) The
parts man said the MB tech at the dealership told him that it allowed fumes to escape from the engine. ????? He could not explain how it
worked. I am wondering, if it is a valve that closes when the engine is turned off, to maintain fuel pressure between the fuel pump and fuel injectors.
So I am down to two possible reasons for my problems.
1. Possible malfunction of the fuel regeneration valve
2. Leaking fuel injection valve

Thanks for any help