1: Work on the Overflow Vent
Cover overflow opening in basin or tub with wet cloth. Most kitchen sinks don't have an overflow vent, but, if you're working on one of two side-by-side basins, plug the other basin's drain opening with wet cloths. In homes that have two bathrooms back to back in adjacent rooms, both may be connected to the same drain. In such cases you must block the other basin at both its drain and overflow vent. Shower facilities seldom have overflow vents; bathtubs do. Cover all of them with wet cloths for plunger to work properly.