Having the same issue. I tried pushback with GSX and through FSX. After both the brakes would not work. I recalibrated, did maintenance, checked failures all were reset and clean. I started the flight from the end of the runway everything works fine. Prior to the brake issue I did adjust the failure rate to 5x but have since put it back to 1x with no effect. I using FSUIPC for controls. Other than brakes having no effect all indications show brakes working normal. My guess is that is its software. Next step is to uninstall GSX and see if that fixes the issue.