I assume you replaced the reg/filter with a like idem,,,
anyway try this
after the car has sat overnight or all day , remove the gas cap, with your ear next to the filler tube ,,,have some one turn the key on,, if the fuel pump runs more than 5 to 10 seconds you have leak down.
this could be the filter/reg or the pump (( or the injectors)) .

when i installed my V8 I removed the filter/regulator,, I have a manual regulator,, and i have to turn my key on for about 5 seconds then start and it starts right away other it will do as you describe ,, I know i have leak down,, i built it that way, you should have 60 to 65 psi on your fuel system even to the next morning after you shut off the engine,, that is why the car starts right away,, hope this helps. jim