My Anti-siphon lessons Learned
I really am not grasping what your question relates to. If you're referring to the check valve/ 180 deg assembly it has but one function. When your engine is running the discharge water from the heat exchanger is passing thru that set of hoses. When the source of water from the Pump/ Exchanger stops the check opens and whatever water is in the line would flow/ drain down and thru the mixing elbow and into the exhaust.
In what state are you getting water flowing out the Anti-siphon line?
I have recent, relavent experience that the Anti-siphon does in fact work. I had a rebuilt/ new-to-me 3HM35F installed last Spring. The fellow who did the install for me/ actually two guys involved, the mechanic and an acquaintance who did the engine bed mods to accomodate the much larger engine/ bellhousing.
Our mechanic friend who had pulled the 2GM20F had not remembered disconnecting the anti-siphon hoses and setting them off to the side. The 3HM35F had come in the crate with a short jumper hose between the heat exchanger water outlet and the mixing elbow. The several weeks elapsed between the time of the 2GM being pulled and him getting back to the install after the glassing work was completed, he obviously forgot the external hosing.
After launch I would guess the engine ran for no more then 15 minutes while we did a check for leaks and then motored over to her Slip. The next day I was on the boat doin' whatever and the mechanic happened along. He wanted to hear the new engine run. Turn key, hit button and only a millisecond of starter sound. I went below to open things up, got a light and pretty soon recognized what was amiss. Flipped the decompression lever set and had him roll the engine over. She cleared herself and started right up. Later checked the sump and no water had gotten into the oil. And did an oil change soon therafter since she was freshly rebuilt.
What had happened, well, exactly what that anti-siphon set-up is meant to guard against. Without the anti-siphon loop installed, after shutdown and cooldown there will be negative pressure created in that relatively short length of hose between the mixing elbow and the ready source of water laying in the wet muffler. Also recognize there's additional volume of water beyond the muffler that's been pushed out there by the force of the exhaust. It's going to drain back into the muffler further raising the height of that little resevoir.
Reconnected the Anti-siphon and haven't had a problem since.
I did have another unrelated engine cooling water experience recently though. I am travelling this year so LAYLAH is home on the Hard for this Season. While home several weeks ago I was anxious to get her running, I'd only rolled the engine over a few times since haul-out last October.
I would usually place a 5 gallon Pail on the sole, fill it with water and put the intake hose off the thruhull into the pail. Wanting to actually warm the engine up/ let it run for 10 or 15 minutes I elected to connect the garden hose directly to the raw water pump. Hooked things up and climbed down to turn on the water. By the time I was back at the ladder there was already water flowing out the exhaust. Climbed into the cockp[it and hit the starter, Nudding! Assuming I'd flooded a cylinder I pinched off the hose and wedged the Decompression Lever Set in the open position. Rolled her over to clear her, released the Decompression and tried the starter again. Took several tries but she was soon running smoothely.
Lesson here is that the Pump impeller will pass water introduced to it at 45-50 psi!