I cannot get my bathroom sink drain to stop leaking, specifically from the doomed rubber gasket area where the threaded tailpiece comes through. The "plumber" at one of the big box stores told me that I couldn't use a pop up drain assembly with the holes for an overflow if my sink doesn't have an overflow? My sink is one of those inexpensive molded one piece vanity tops and does not have an overflow. He said I couldn't use a pop up type drain as they are made for use with sinks with an overflow or it will certainly leak?
The gasket that connects to the bottom of the sink can be a tough repair, if the mating surface the gasket contacts against is rough or damaged the mating surface will have to be repaired or in extreme cases the sink replaced. Most bathroom sinks have overflows. The overflow is usually a small hole about one inch from the top of the bathroom sink. If your sink does not have a overflow there is many drain assembly available that can be installed as well as pop up drain assembly with some modifications.