Low fuel pressure, new almost everything ! Help

Hello A body freaks,

Referring to a 74 Duster, 360, Holley DP 600 M1 intake, mechanical fuel pump.

Car started stalling out about a month ago, wouldn't idle consistently, and I noticed low fuel pressure if it did around 2psi. Typically It's running around 6-7. I have a gage on the fuel rail that I bought from Summit.

So I changed the fuel pump ( mechanical ), and filter ( Looks like a gold stone top hat inside the Summit fuel Rod before the gage ). And the car seemed to run fine for a about a week.

Then same thing happened, car needed constant pumping on the gas to keep it idling, and noticed low pressure again at the fuel rail.

So I decided to pull the tank, and change it, as well as the sending unit, and fuel line. I notice the sending unit had no sock on it, and the plastic tube on the end was cracking and disintegrating badly. Tank looked pretty crusty inside. So new tank, sending unit, fuel line all the way up to the pump.

Started it up tonight and pretty much the same thing, low pressure ~ 2psi at the fuel rail at idle.

Any thoughts or ideas ? Why did my idle pressure go from 6-7 psi, down to 2psi after changing almost every component ? The only thing that didn't change was the hose after the pump to the fuel rail, and the fuel rail itself.