Electric trailer brakes are normally engaged only when the tow vehicle is decellerating. Trailer surge brakes need the back-up light disengagement when reversing.
Trailer surge brakes have a coupler on the trailer tongue that floats front to back. The coupler is connected to a hydraulic brake master cylinder, and it's spring loaded so it's normally not engaging the brakes. When the tow vehicle slows the trailer surges ahead, the coupler moves against the springs along the tongue activating the master cylinder, and the trailer hydraulic brakes are applied. The problem is that when backing uphill the same thing happens and the brakes are applied when they aren't wanted. The back-up light solenoid locks out the trailer brakes when the tow vehicle is in reverse.
I like roadrutz's work-around.
One time I was in my Tundra towing a U-Haul vehicle trailer with a VW Passat aboard. The route included a long downhill on a fairly steep mountain. I could really feel when the surge brakes activated as I braked down that hill. Got home fine. The next day a stainless brake hose blew out on the Tundra. If that had happened on that downgrade I'd have only the engine and parking brake to get slowed. Whew. Never again will I buy brake hoses with a Teflon inner liner. Teflon doesn't have the flex fatigue strength needed.
