1993 Plymouth Acclaim 4 cyl

we are fixing my car at the monent. were replacing the gas tank. we got the old tank off and started moving the parts from the old ones to the new ones i had to replace the fuel pump...but some of the other parts were still good..but we ran into a problem the i belive its called the backflow valve its on the top of the tank how do you but that in..we have tryed like everything..