Fuel pressure dropping after shutting off

[SamW]

Owner of a 4.6is for a couple of months now and decided to ask for some tips here.

The initial issue was the car running lean, hesitation to start, lack of power and some intermittent stalling after start up. The cause was finally narrowed down to the fuel system. After shutting down the car the fuel pressure drops almost immediately. While compressing the fuel line the pressure stayed constant. I think it would narrow it down to the check valve leaking somewhere.

My question is related to the check valve's location, is it in the fuel pump? And if it is, should I get only the actual electronic pump replaced or the whole delivery unit? And what about the pressure regulator in the fuel filter, could it cause similar issues? Or is there any other potential valve's which could cause this kind of problem?

Thanks is advance!