Just Put New Bathroom Sink Top In And Seem To Have A Problem With Water Leaking From The Stopper Valve--we Fill Up Sink Then Let It Out And Water Keeps Coming Out Where The Stopper Valve Is--have Tightened It And Tried Two New Stopper Valves Still Keeps Leaking

First of all you're going to hafta define "stopper valve"for me. Are you referring to a stopper that leaks water down the drain when closed or the pop-up assembly under the lavatory? Does it leak where the rod goes into the drain? If this is the case there's a small nylon washer that fits ahead of the ball and fits into the metal seat. If that washer's missing you have located your leak. Good luck, Tom