A water leak and water siphoning out of the washer are two different things.
Water leaking out would end up on the floor. Water siphoning out of the washer ends up going out the drain hose. You won't see any water on the floor if the water is being siphoned out of the washer.
if the pressure switch would stop the fill cycle why wouldn't it stop the rinse cycle? I've stopped it from filling from the rinse when it reaches about 3/4's full of water on the rinse.
If the washer fill's to 3/4's full it's probably not a problem with it siphoning.
What happens if you don't stop it from filling during the rinse cycle? Does the washer over fill to the top of the washer if you don't stop it?