Vented loop air valve
A properly functioning vented loop should have a one-way air valve that only allows air IN--to break a siphon...it shouldn't allow air or anything else to escape. In some loops, the valve is internal...when it fails, the whole loop has to be replaced. On most, the air valve threads into the nipple on the top of the loop. Since they're shipped separately, they're often missing in the loops on the shelves in stores...most people don't even know there's supposed to be a valve with it, so when they see waste squirt out of it, they try to clamp a vent line on it...but because the nipple is very short and hard to get a clamp on, that's usually not a successful way to prevent odor from inside the hose from leaking out around the vent hose each time the toilet is flushed. The solution to your problem is a new air valve.
