Rick, you misunderstood this part:
Water coming into the bowl in the dry position is due most likely to worn gasket or faulty spring or both as Ron mentioned.
Water cannot get into the bowl unless the water is in a line connected connected to the bowl. So SOMETHING is happening at the thru-hull that allows water into the head intake line.
They recommend putting the antisiphon between the bowl and the holding tank...
How would a vented loop (antisiphon device) in the toilet's DISCHARGE line prevent sea water from getting into the bowl via the head intake line? A loop--just a loop, not a vented loop because no siphon could get started in that line that can't be broken by simply letting out the sheet to right the boat--can prevent backflow from the tank that spills into the head discharge line when the boat is heeled from getting back to the toilet.
...however some put between the head pump and the bowl.
"Some??" A vented loop is ESSENTIAL in the intake--and it MUST be installed between the pump and the bowl, not between the thru-hull and the pump--in any installation in which the toilet is at or below waterline. But a VENTED loop would be recommended in the toilet DISCHARGE line ONLY if the toilet flushes directly overboard. Without one, sea water CAN flood the bowl if the discharge seacock is open while underway.
However, vented loops only do two things...1. create an arch high enough above the waterline to prevent water from outside the boat from getting over it while the boat is at rest...and 2. break a siphon. Siphons can only be started by PULLING water through a line...which is what priming a pump does. Pushing water through a line will not start a siphon. So a vented loop CANNOT prevent an effect known as "ram water"--sea water PUSHED ("rammed") up a hose through an open seacock--from flooding and even overflowing the bowl. So the air valve in your vented loop may be just fine. Otoh, if some misguided soul put a vent LINE on the nipple instead of an air VALVE in the hole in the nipple, it does need repair...that vent line needs to come out! 'Cuz the diameter of that hose is so small that it only takes about a week for salt and sea water minerals to clog it up...turning a vented loop into an UNvented loop that no longer has any ability to break a siphon.
Bottom line: No matter what else may be causing your toilet bowl to take on water, it couldn't happen unless water is getting into the head intake line through an open or leaking seacock and then being pulled over the top of the loop. Replacing gaskets and springs may keep it out of the bowl, but that won't keep it out of the intake line. So you need to correct the problem at SOURCE of the water (the thru-hull) before it becomes a much bigger problem.
And btw..keeping the intake seacock closed all the time and flushing only by pouring water into the bowl is VERY bad for the pump because bowl contents only go through the bottom part of the pump...leaving the top half of the pump dry all the time, which wears out the rubber parts a lot faster. Replumb it to tee it into the sink drain line and operate your toilet as it was designed to be operated.
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