Fuel PSI Problem

[dmcclanahan]

I have a 1974 C10 with a 1988 350 which has a carburetor on it. My problem I'm having is, there is still pressure in my fuel line after the truck is shut off. I can see the pressure because I have a PSI gauge just before my carb. My main tank is need to be cleaned out (it is located on the passenger side), so I'm using my auxiliary tank as my main tank now (which is on the driver side of the truck).
-I just installed a new electric fuel pump and two new fuel filters.
-The pump is rated to have 5-9 PSI.
-The auxiliary tank has a filler nick and one outlet at the bottom of the tank
And for some reason the PSI gauge holds at 6 PSI after it has been shut off. An hour or two go by or when I remember to go check it, it has gone back to 0. The pump I had before did not do that (which was connected to the main fuel tank). I know that I need to clean my main tank but for the mean time Im using my auxiliary one. Any idea's why this is happening?