I went through this…. First, make sure you have a fuel pressure gauge, because just watching fuel squirt out of the shraeder valve on the fuel rail is not an accurate way to monitor fuel pressure.
Then, if you are still getting quick bleed down of fuel pressure, problem could be the cheap check valve inside the fuel pump. I had this issue with a brand new fuel pump, and it was quite puzzling. To diagnose this issue, you might need to remove the fuel tank to get to the fuel pump. I have created a fuel pump access panel, so removing the fuel pump was easier. With the fuel pump removed, I could blow back into the fuel pump outlet without restriction, and confirm that this check valve was not working. The 10 cent rubber check valve might be known as the “fuel pressure regulator”.
For the fuel rail unit known as the fuel pressure regulator/dampener, I do not think you will be able to locate a new replacement. Salvage parts would probably be the best option. If not, I found a Volvo regulator/dampener that worked as a substitute on the 4.0 SOHC. I do not recall the part number, but you can find it on this Forum and/or discussion on the Internet. Then had to take the new replacement unit and the old lines to a hydraulic fabricator and get a custom replacement line created. It worked, but this was a lot of running around.
Good luck!