are you nuts shaman there are no vented loops in a fresh water system.
i would open up every hatch in the boat and
follow the fresh water lines drawing it as you go. fill the fresh water tanks. check for
a valve in the line. if there is a valve near the presure water hookup, then turn it off. if there is one near the tank then turn it on. just to make sure check all hose clamps to see that they are tight, and not sucking air.
by the way vented loops are for sea water intakes and below wapter discharces. they are
used to stop water from syphening back into the boat. so therefor ventedloops are un needed in a fresh water system.
now strainers have a tendency to leak air. and are very comon in fresh water systems, before the pump causing it not to prime.