So I get a call on a bathroom sink not draining. When home owner flushs toilet, sink backs up or in the morning they will find sink full of water and can't figure where it's coming from? This is a one story home with bath and sink draining to same line which flows to main stack in bathroom. So I get their and flush toilet and sure enough sink fills up so I look at bath tub and no water in tub and drains with no problem and this drains to same line as sinks drain to. Seeing as sinks are 3 ft. higher then drain in tub I can't figure when you flush toilet their is water in sink but not tub? I did fix the problem but wonder why sink filled when flushing toilet.