I owned a King Duo Gravis in the 1970s that was like this. I had my Navy band instrument repairman silver solder a bar to the F trigger to make both valves open when needed. The F key could be pressed at will, but rotating the thumb forward would activate the second valve key to bring in the extra plumbing to put a D in 1st position. Without the extra bar on underside of the F key it was possible for the F valve to close slightly when rolling my thumb forward to the second valve key to bring in the D tuning, which destroyed the free airflow. Great playing horn once that mod was in place!