Sorry for the confusion regading acft.cfg files. It dawned on me afterwards that you are referring to the "Customize Aircraft" option under the "Add-ons / GSX" menu. I did disable the "dummy pushback" option on the menu and that did clear up the problem. I do now get a small bit of vibration during the PMDG777 pushback, but that is preferable to the constant break tapping that I encountered previously.