I did some work on mine. There's not too much that you can do. If you look at the link below, you'll find a picture of the trigger group. You'll see a little "V" shaped spring. This is the key. If you make a replacement spring (piano/music wire- hobby store) like this, but out of smaller wire with a couple of extra turns, you'll get a lighter trigger pull. If I remember correctly, I used 5 turns(instead of 3) of a wire about 2/3 the thickness of the original. My trigger pull is now about 2 lbs.http://www.network54.com/Forum/113813/thread/1155358038/Need+TF-99+%28QB36-2%29+trigger+diagram
It also helps if you polish the piston around the area where the sear latches into place.Also, both the upper & lower surfaces of the sear (made from metal leaf riveted together) can be smoothed out.Please ensure you do not round off any edges or corners.