I feel your pain! Been there, done that with my 4-108. It was driving me crazy.
One day, after bleeding and checking and backing and filling I was pumping the little lever on the fuel pump
....heard a little whoosh, whoosh.
It was very quiet in the marina or I would never have heard it.
Turned out to be a pinhole in the BOTTOM of the copper fuel line. This line ran low in the bilge
, was often in water
from the occasional stuffing box leak, and had corroded. Found it with my fingers (stopping the whoosh whoosh sound); it wasn't visible.
Replaced the copper line with a high-quality fuel hose. No problems since (about 15 years).
Good luck...it's sure to be an air leak somewhere.