We would begin troubleshooting by unplugging the brake lever switch wires from the controller and seeing if it works then. It could be possible that one of the brake switches is bad and staying in the on position.
If that does not help then please let us know and we can take it from there.
We would start by testing the input Voltage at the new controller's battery connector to make sure it is getting Voltage and the right amount of Voltage from the battery pack. If that looks good then we would check continuity of the power switch and make sure it is properly connected to the controller.