My 87 300SDL has a lot of air in the inline filter(only half full). So I I put a new inline and spin on filter. Swapped supply and return fuel line in case of clogged tank screen. Still only half full and no power. I do not see any fuel leeks or weeping lines. I am running a 50/50 blend of diesel/bio. Is there a way to test the lift pump? Any suggestions would be greatly appreciated.
Thank you,
A

__________________
1987 300SDL
187,000 miles