I'm not sure I can help you with this one. Maybe someone else can. For clarification, the lav basin doesn't retain the water when the pop up is engaged in order to fill up the lav basin with water is what I'm guessing that you are asking. And you replaced the old o-ring on the pop up with a new o-ring and the water slowly drains out but the water is not leaking under the cabinet. If the water was leaking under the cabinet I would say that the plumbers putty for the drain needs to be repacked but what I think you are saying is that the o-ring is not holding water in which case I'm not sure but maybe a thicker o-ring is needed.