I have a 1996 F-150 with a 4.9. I am getting P0171 and P0174 codes. Additionally, I find that gas from the front tank is being pumped to the rear tank. I have not been using the front tank because I do not want the overfill the rear tank. Based on what I have read in the forum, I assume that the fuel pump and/or check valve is faulty in the rear tank. Is the check valve in question a part of the fuel pump or do I have to buy the whole assembly? Thanks in advance for your help!