80 ford courier fuel system problem(low)

[tickyul]

Ok here is one I need help on.my courier is supposed to put out between 2.8 and 3.6 lbs of fuel pressure.It is only putting out 2lbs max and will immediately drop that to zero (wont hold any reserve)when the key is shut off.I have found no fuel leaks on either the pump or vaccum side of the fuel pump.I replaced the rubber fuel lines just in case but am still low.I also checked the needle and seat assembly in the carb and it is fine(new).When I plug the fuel return line the fuel pressure goes up to 2.5-2.7.I think that my problem is twofold but I dont know how to test either(not the fuel pump it is new).I think the check valve on the fuel line is bad not allowing it to hold pressure,and I think the sock in the fuel tank is probably dirty further limiting pump pressure.How do I test these 2 items(sock and check valve).When I hit the gas really hard the thing really coughs and spits and the pressure goes down to about 1-1.5lbs.

thanks
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