I had a problem with my brakes not getting power. It was intermittent. After much hair pulling and some parts replacement, I discovered that the brake wire going to the truck's umbilical connector had suffered some road damage and the copper inside the wire jacket had parted. On a Ford pickup with OEM connector, the brake wire is the bottom-most wire leading into the connector. I disassembled the connector, stripped to good wire and reassembled.
You can check the connector with a meter or pull the breakaway pin on the emergency switch and compare voltages.