e53 4.4 fuel pressure problem

[candrade]

Hello everyone. new to the forum. i have a e53 4.4 year 2000 and it has a problem at cold starts. cranks long and then starts. no problem when hot.
i changed fuel pump, syphon pump doesnt leak and the syphon one way valve isn't stuck.
when the car is cold, i press on the relief valve on the fuel rail and doesn't have pressure. looses the pressure overnight.
what can be causing this? is this a fuel pressure regulator issue? should i change the fuel filter w/regulator?
thank you.