MartinE

We have side-by-side two sinks in our kitchen. One has an Inskinkerator and the other a normal drain. There is an air admittance valve fitted to the pipework under the sink. Recently, the normal sink has started to drain very, very slowly. A friend suggested that the air admittance valve might need replacing. I replaced it with a FloPlast AV40 Air Admittance Valve When it is done up tightly on the pipe, the sink won't drain. If I ease off the screw to let air in, it drains normally. I thought the valve was supposed to work when tight. What am I doing wrong?