Tom,
I found the Gary F. thread and the switchover valves, all conveniently numbered 1 thru 5. An interesting thing is that in doing the search for information on my specific problem, I have not gotten any hits on leg vent problems -- plenty on center, side, and defrost vents though. Maybe this indicates that the leg vent design was better than the other vents. That or people just don't get worked up over lack of heat around the feet.

Dan