Keyboard Assignment

I know there is probably something I'm missing here but:I have tried to assign new keyboard commands to the pushback feature. "U" for straight back, "I" for tail left, and "O" for tail right. The "I" and "O" have worked fine, but not the "U". It apparently is used in a "panel switcher" command and pops up the radio stack when "U" is pressed. So, I went to the "panel switcher" menu and only found 1 item which used the "U". From memory (as I am not at home on the PMDG computer), it seems this command used Shift+U. Anyway, I deleted that command and then set up the "U" for straight pushback. Works for a while, but then reverts to popping up the radio stack on subsequent use even though I save my changes. What am I doing wrong or, does anyone have any other keyboard assignment themes I can use for these 3 pushback directions? I liked "U" "I" "O" because they are side by side, but I just want to find what will work.Thanks