Under voltage causes this as battery management cuts in so to over come the park brake remove the two 30 amp fuses located in the osr luggage compartment then refit after 10 seconds then go to park brake with vag com and reset the basic settings which will recalibrate the the callipers,also consider the brakes are locked from sticky pads like normal handbrakes which stick after long periods of engagement,there is no need to reverse the polarity unless the calliper have failed which is highly unlikely and electric park brake is very efficient and failures are very rare.If you have Audi assist support they will come out and resolve the issue

read the link and follow as per safety instruction but also charge the battery for at least 8 hours or check on the mmi screen user settings the battery display if its not listed add 32 to adpation channel 2 in address 07 then restart the mmi and it will be listed

http://wiki.ross-tech.com/index.php/...#Basic_Setting


YOU CAN SOMETIMES OVERCOME THIS BY SIMPLY RECHARGING THE BATTERY OR RUNNING THE ENGINE FOR A WHILE UNTIL THE BATTERY MANAGER STARTS TO SEE CRITICAL VOLTAGES RETURNED AS THE MANAGER TURNS CONSUMERS OFF TO ALLOW CRANKING FOR EXAMPLE KEY LESS ENTRY IS NORMALLY TURNED OFF BY DISABLING THE DOOR HANDLES SWITCHES