We recently gutted our kitchen and have a new sink installed - the old sink was a bit slow but now we have a much worse problem. This sink is located over a walk-out basement bath and that bath drains just fine - all fixtures. My husband has installed all new pvc pipe under the sink and there are no blockages in that area. We have tried drain cleaner and plunging. He just installed an under the sink vent that ratings say works fine and he followed instructions. this was because it appears the sink was not vented with a stack (and there is no decent way to add one now without tearing up new wallboard and putting holes in a new roof - major! but the old one did drain for the most part - would be slow at times and we would use drain cleaner. this now simply fills up within a few seconds and starts filling the sink and then takes a while to finally empty. I will add that he redid a lot of plumbing in the basement and replaced the tub down there with a whirlpool with no problems so he is not a total amateur. We just can not figure out why this problem persists when everything below it drains fine. does anyone have any idea what could be causing it?

Is there a trap for the sink below the floor ? Is there a trap for the sink in the cabinet ? If we could see a picture of the drainage under the sink we might be able to see whats wrong.

There are so many possibilities that if we tried to give suggestions it would just confuse you if they did not pertain to YOUR problem.