The large piece under the sink is leaking. Have tried the gasket that is included and plumbers putty. Not sure where to go now. Model #456 365 Lyndhurst model.

Any help greatly appreciated

Make sure the gasket under the sink sits properly inside the hole in the sink and the nut is tighten up real well. Also, apply teflon tape on threads of the tail piece pipe. Thirdly, make sure the pivot seat, washer and nut are assembled in order as shown. Here is exploded view of drain with pop-up assembly. Let me know how you did. Milo