This is the setup - sorry if it sounds simplistic - I am learning
. The raw water
intake goes to the pump, another hose comes out of the pump and goes up to a plastic loop with a vent on top, then back down to the toilet bowl. The waste discharge goes through the pump/joker valve through a much larger hose to a Y valve, then to the holding tank
which is relatively close by 3.5-4ft. Off the tank is a hose to a macerator and a thru hull
that's never used and a clear vent hose that leads to the transom. There are no other loops. The vent hose does have a dip as it goes under the sole, then rises gradually when it gets to the lazarette to the vent. It's currently a bare hose end sticking out of a large lazarette vent b/c I haven't been able to fix the corroded connection to the old vent yet and didn't want the lazarette smelling like poo (which is clearly does when someone pumps the head).
I discovered this morning that when I am pumping seawater into the bowl, I hear the vent on the top of the loop suck air. I almost always use fresh water
from a jug to flush urine so I don't hear it then. Now that I understand this is all before the bowl I guess that makes sense.
I routinely pump 5 bowls of hot fresh water
through while I'm at the pump out with no resistance - pretty aggressive pumping. The only time I've had water not want to go through was this morning after allowing vinegar to sit in the lines all night for whatever that's worth. Working fine now.
So it would seem there's resistance to anything other than pure liquid. I'm not taking anything apart until I go to the pump out which will be this weekend, and I'm going to try the Head
Mistress' muriatic acid test, so I will keep you posted.
Thanks everyone for their ideas - I'm learning