Waited 6 weeks for an answer on this one. We were told that the reason, the water level in our toilet keeps going down to like 3 inches after it is flushed, is because our air vent is blocked. We live in a Mobile home, and can not find one air vent going thru the roof and only one under the sink in kitchen. Are they in the walls? This is a regular toilet, about 15 years old, works great, except within an hour or less, the water level is way down.