This thing should lock the tires up when its empty and it won't
Yessir, I agree. Like Chuckdogg posted, with a conventional controller, the more brake application, the more voltage. I'd have someone in the cab slide the manual application lever to the max braking position and see what you get then. It's been so long since I've checked one I can't remember what the reading usually is but it will be more that just 2+ volts. It's common for the trailer brake wires to break where they run from the frame down to the axle or backing plate due to constant flexing, you might take a look there. Also check your magnets, grease on their contact plates, etc. When you adjusted the brakes did you turn the adjusting wheel tight enough to lock the drum up, then back the wheel off? That centers the shoes on the backing plate first in relation to the drum so you get an accurate adjustment as you then back the adjusting wheel back off. If you just go tight enough until you hear a shoe scrape that's not an accurate adjustment (on any drum brakes).
If this is on the Ford integrated brake controller you have to be traveling at least 15 mph (I believe)
Seems odd they'd design it that way. There's been lots of times I've had to creep down a steep rocky road with a loaded horse trailer at WAY less than 15MPH and I definitely needed and used my trailer brakes.
