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380 kpa or 55 psi
Normally if the pressure reading is low this indicates you have a weak fuel pump. The pressure regulator normally will only change the pressure output if the vehicle is running. So you can check and verify the voltage on the fuel pump is good and the ground. If they check out and you have no restrictions in the fuel line than replacement of the fuel pump is definitely in order to correct the issue.
Let me know if you have any other questions, If I can help in anyway I will. Please take a moment to leave a positive rating or accept the answer. Thank you very much. Charlie.