I really question the controller at this point. It really does sound like the focal point. It is the only thing providing electricity to the brakes when the TV is applying brakes, if I understand your description of the behavior. What is the model of controller you are using?
The break-away switch can be replaced easily for $25. The brake controller can be replaced for $120 with a good Prodigy, as needed.
If you haven't already explored the brake system wiring on your PUP, it is pretty basic when laid out.
Electric power for the brakes (electromagnets inside each brake, thus the need for electricity) comes from two sources: the TV's brake controller and 12v system, and the break-away switch with 12v from a battery on the PUP (either the main battery or a small, dedicated battery for the brakes).
When the pin is pulled out of the break-away switch, this completes the circuit and allows the full voltage to activate the brakes. There will be no intermediate levels of braking - it will be full voltage/power and immediate. The wires leading to and from the break-away switch only carry the positive charge: one side comes from the positive terminal of your PUP's battery, the other side goes to the brakes. However, using the TV's brakes will have no affect on this system. Only when the pin is pulled out will it activate.
The brake controller in the TV, as you are acquainted with from your description, provide proportional power to the PUP's brakes through the 7-way connector. As you mention, you can dial this up or down. Activating the level partially should apply some voltage and more voltage is applied as you move the lever further to max.
What happens when you slowly activate the manual lever on the brake controller? Does it lock up or is gradual braking applied?
As opnspaces asks, if the break-away switch is not connected to the battery, do you still have the problem? If so, it should have nothing to do with the break-away switch at this point. It's in the controller and/or wiring.
And Terryallan hits upon what I think is the culprit, in that the TV's brake controller was connected when the break-away switch was activated, sending an electrical charge back into the controller, likely scrambling it.
Let us know how you progress, so we can all learn. And good luck hunting this down. Like I said, there's only a few components to deal with so it's a matter of ruling them out.
