"My new house had 2 of these fitted on a double kitchen sink. Now after 2 years of use the Air Admittance Valve on the side that we regularly fill up with water leaks water badly when you dump more than about 3" water from the sink. The one that we rarely fill with water still passes the test. Cleaning the trap and valve has only made matters worse! It all looks perfect except for a bit of barely detectable distortion of the thin black rubber washer in the AAV and the valve has too little weight and no spring to flatten it against the seal face. If it doesn't seal it allows water to come up the duct from the trap and a jet of water hits the valve seat sideways and further displaced it. I am going to seal it up with silicone and suffer the gurgling and reduced water seal height. If the valve doesn't seal you get a few sewer smells anyway!"