My old baseboard heater has a pipe on the very end. There is a screw on top that was open and mildly leaking so we closed it. Now, there is some sort of white plastic thing, probably a valve that is flush with the pipe and is mildly leaking. How can I stop the leak as economically as possible? We're selling the house. Thank you.

We need to know EXACTLY what the part number is on the plastic part that is leaking or post a picture.