air leak or worn lift-pump?

Aloha,
our 83' 300SD runs excellent on the SvO conversion ,new Standine filter,prefilter,double filtration of non hydrogenated soyoil ...for 700 mls. Then slowly looses power uphill.I know about the booster pump.. primer pump does prime fine.I was told that the lift-pump could have gone bad.Or we pulling air somewhere.how do I test that before I oder a new lift-pump?