"The grip buttons were not working to brake in the game, but I realized that pulling back on the joysticks would apply the brakes. To enable braking as intended on the grip buttons, I removed the old actions from the palm triggers and instead added a "Use as set analog action" type of event, and then selected the left/right Second Trackpad/Thumbstick Value, and set the X value to 0 and the Y value to -1. With this setup holding the grip buttons now applies the mid-air brakes!"