Tough bathroom faucet leak

Difficult to fix  under the sink faucet leak, replaced all the hoses being carful not to over tighten. Water drips from the lever that's connected to the stopper mechanism, and might be coming from the "Mixing Chamber" for lack of a better description. ie,  down the 1 1/4" threaded pipe that goes up to the spout. 

Is it possible to have a leak from there? Turns out, both sinks in that bathroom have the same problem. Maybe a gasket gone bad after 15 years?

Thanks for any help.

Did you use pipe tape.?????? If so..

Make sure that everything is seated properly, if not, re seat it so the threads align tighten and try again.

If seated properly try tightening it a little more then hand tight. I have have went as far as tightening it pretty good with a wrench. 

If it still leaks take it apart completely and replace what you are calling the gaskets.

Examine the exposed areas for deteriorating gaskets or corrosion. These factors prevent a proper seal. Use a butter knife to remove the old gaskets. Dip the steel wool in white vinegar. With the steel wool, clean the cavity where the gaskets were seated. The vinegar dissolves the mineralization as the steel wool breaks loose any corrosion or debris.