By "leaks when shut off" do you mean around the stem or it just does not fully shut off?
Some (most?) rely on the piercing pin-to-pipe contact to make the seal when closed so if they were cranked to bottom out, you will never get a good seal. IMHO they are designed to be open most if not all of the time. If the top half of the saddle has a pipe thread into which the valve screws, you might be able to find a better quality valve that does not rely on the piercing pin to seal.
Places that sell hydronic heating supplies might be a better place to find replacement parts or fittings.