[JRTJH]

It almost sounds like you wired your brakes to the 12VDC hot line in the trailer umbilical rather than to the brake lead from the brake controller. The breakaway switch is connected to the same brake wiring, but is not the source for routine braking. The breakaway switch is the ONLY "always hot" lead to the brakes and that power source is "interrupted" at the switch by the pull pin that is connected to the tow vehicle. You should only have "12 VDC power" if the pin is pulled from the breakaway switch or if you activate the tow vehicle brakes and the brake controller applies power through the umbilical on pin #2. Otherwise the brakes should be "unpowered". If you connected a new line from the brake wiring you installed to a 12VDC "always hot" source, it sounds like you've "hot wired" your brakes and eliminated the brake controller circuit in the truck and at the umbilical connector (pin #2).

You might want to check the wiring that you installed and routed. I think you'll probably find the problem is in how you connected it to the umbilical wiring at the front of the trailer. Hopefully it'll be something as simple as that. If you need a umbilical wiring diagram to be sure it's wired properly, this is the one that's commonly used on all 7-pin connectors. From the connector to the junction box, it will be like this diagram. Once it enters the junction box, well, that's where you really need to pay attention to how things are connected. it's very easy to put a wire in the wrong "bundle" and cause the kinds of problems you're facing.

Let us know how it is going and what you find.