First clean the metal parts thoroughly using denatured alcohol, or carbon tetrachloride, and a toothbrush. You want the parts to be bare. Who knows what someone put on them that may now be gumming up the works!
I am not sure I quite Understand what you say is not functioning correctly. If the trigger assembly is removed from the action, you should be able to set the rear trigger, and then fire the front trigger and see both levers move. The spring should pop the firing lever up fairly quickly. Is the rubbing in the set trigger, or in the forward, main trigger?
If instead the problem is that the hammer doesn't always fall fully forward, but catches on the half **** notch, then check the " fly ". This is a small, triangular shaped piece of metal that is attached to the tumbler. The tumbler is connected to, and holds the hammer. The "fly" is held to the tumbler by the bridle that surrounds the tumbler, and the sear bar. Be very careful gaining access to the " fly ". It is so small that it tends to " fly" off into space, and is often lost. That is why the part has been called the " fly "! I have found that the fly often sticks because oil has jelled around it. All it needs is a very good cleaning. In other guns, the surfaces of the fly and the tumbler are coarse, and can benefit from being smoothed with a fine stone. Since you have tuned the actions on other handguns, I am sure this is a simple matter for you to do yourself.
You might be helped by going to Bob Spenser's Website, Black Powder Notebook
http://members.aye.net/~bspen/index.html
And looking at the pictures of the inside of locks that he has in a couple of articles there. All the parts are identified by name, so this won't be difficult to understand. You may need a spring vise to remove the mainspring in the lock to do the work I have mentioned, if it is the " fly " that is the source of the problem. If you **** the main spring by pulling the hammer all the way back to full ****, you can clamp the mainspring, in that position, then releast the hammer to allow the mainspring to removed, using a spring clamp, or other tools. I use a vise grip, and simply use some masking tape on the jaws to keep from scoring the surface of the spring arms with the jaws.
