From what I remember that system that had about 55 psi with the eng running and the vacum line hookede to the fuel press regulator. Pull the vacum line on the regulator and it would jump about 10 psi to about 65 and pinch the regulator return line line and instantly it would jump to about 100 and I mean right away. Seems to me even the non turbo systems like the V/6 mini vans that ran a system of about 38 to 48 psi would jump right to about 80 or more as soon as the return line was pinched. Thats about what I remember but its been years as Mopar went to the returnless EFI systems in the 90's on many systems. But if the press does not jump as soon as you pinch off the return line it sounds like the pump is weak. Is it weak enough to be your problem ?? Thats the question and as I said I have not worked on them for many years but it is more important for the reserve fuel pressure on the turbo cars. Ron