I have a full size Bronco which I use to tow my 21' Striper with a 150 HP. I always back down the ramp in 4wd. If I do not the boat and trailer weight pull the tow vehicle backwards down the ramp. The nature of the disc system with a surge coupler means I have no trailer brakes ever when in reverse due the solonoid releasing fluid pressure to the brakes. Is this correct or do I have it wrong? The Bronco has good size tire with new tread. It does not seem right that I should need to use 4wd as I see other smaller tow vehicles not sliding. I have plenty of tongue weight also. Is this one of the differences between disc and drum surge activated brakes on boat trailers?