1988 gq patrol 4.2 petrol

i have been having fuel issues
replaced carbie
replaced fuel filter
replaced macanical fuel pump
the peoblem i have is the pump wont pick up any fuel from the fuel tank so far it has taken 2days and 2 flat batteries trying to start the gq
tomorrow i will be changing the fuel filter again as i think there is a problem with it as it wont hold any fuel in it .it drains back into the tank
this problem is doing my head in as i am running out of ideas

any help would be greatly welcomed as this gq is my pride and joy
the 4x4 is a 1988 4.2 petrol

cheers mark