"As I understand Harvey's proposal, he will keep the surge brakes on the rear axle. If he has a long down grade, he will put the pin back in the surge coupler, to disable the surge brakes, and possible overheating of the disc/hub/grease. There is no controller for surge brakes, they only work by the truck braking (engine or brakes), and the forward pressure of the weight of the trailer allows the coupler to slide forward, putting pressure in the hydraulic cylinder, thus slowing the trailer.
The in cab controller is controlled by the truck's braking system, or can be controlled manually. For any load over 6800# in parts of Canada a system which has in cab braking control is mandatory. The electro magnetic brakes will be used by Harvey when descending grades. When he is back on the level, and not backing, the lock out pin on the surge blade coupler will be removed and the surge brakes will then be in play.
Can it be done? Certainly. The issue I see is the forgetting to put the pin in or pull the pin out. Maybe a red flag on the steering wheel as a reminder????"