I have a 2005 Pacific dual axle trailer with surge disc brakes for my Parker 2320. The problem I am having is the trailer brakes lock up when backing up. It happens on level ground and is worse when backing up hill. I've tried backing up with the trailer unplugged from the tow vehicle and the problem still remains. The trailer is in good condition, as I wash it down every time I use it. Any ideas or input is appreciated.