whats up guys..new to this forum..my dad has a 94 f150. there was a problem with the front tank and he would use the rear..but the return fuel goes into the front. he felt it was the fuel pump so we swapped it out(got one at the junkyard). yesterday it choked out (running on the front) and didnt want to start. today its running. but wondering why gas was only returning to front tank? bad check valves ive heard. but dont know exactly where they are.. any other insight is greatly appreciated.

 

check valves are in the fuel pump sending unit assembly...it it integrated as one unit