I recently installed a new sink in the kitchen. Previously it was a dual drain and now is a single drain in the center of the new sink. I now have an issue where the water doesn't drain all the time. It isn't backed up down the line because I can stick a knife or spoon into the water and air will then bubble up and it will all drain. I can also turn on the disposal and not have the water drain. I never need to plunge to get the water to drain just "pop" the air bubble. Because of this it seems like it is an air pocket that is holding back the water from draining. I do not have an air valve installed previous to or after the new sink install. Is this the issue? If so, why wasn't an air valve necessary before the new sink?