I have an emco shower faucet that is leaking cold water after I changed the seat and the stem on the cold side and rebuilt the stem on the hot side. What else can I do?

You say you changed the seat and stem. Do you mean seat or washer. If it was just a new washer did you check the seat that the washer seals to? Stick your pinky into the hole and feel for burrs, if you find some buy/rent a reseating tool and reseat the valve. If I misuderstood what you were saying I apologize.

I would try changing the other seat as well. Are you using pipe dope on the threads for the seats? Are they going in smoothly?