I have a lister engine on boat and it is sucking air somewhere and i dont know where?

it is been like it for while so i have completeley changed all fuel lines and filters and it is slill doing it. i think it is something to do with the new filter i have put on as i cant see air bubbles in the fuel line b4 the filter but i can see air in the line after the filter. but all connections are tight and i just cant see a possible way air can be getting in but it is somehow. any ideas