fuel pressure reg actin funny

[littlevictories]

So I just swapped intakes, fuel rails, injectors, and FPR on my 2002 DBC flex fuel 5.3. -kept the original return style fuel crossover . the intake and rails are also from a flex fuel truck but DBW.

I was trying to cure a possible vacuum leak.

Now the truck won't prime up to 50psi with the key in run. The pump primes, but the pressure at the rail quickly drops back to zero.

To start it, you have to crank for ~5 sec and then it fires. Before it would fire instantly.

When you shut it off and you can hear it whistle/gurgle down.

I swapped FPR's to the old one, same game. I'm worried that the drivers side rail i swapped in is messed up.

Any ideas on where to look after that?