Chris: I have read an technical article in a railway magazine about the Thurbo GTW. The airbreak is there graduated combined with the electric break. The dynamic break act first, then blends the airbreak in. Dont know if it is the same in the other GTW.
This is how the brakes are right now in my GTW. As you slow down the air brake is blended in automatically at slower speeds (as long as you have not overridden it with the pneumatic brake handle). If you have moved the pneumatic brake handle away from the 'neutral' position then the blending doesn't happen as the script assumes you are manually controlling the air brakes.