Originally Posted by bill_de
I never thought about what happens in reverse. What does the extra wire activate that can deactivate the plunger?
I towed a 2,500 lb. sailboat with surge brakes on the trailer over a ten year period. There was no mechanism to deactivate the brake in reverse (perhaps newer rigs have a deactivating device). But, given the weight transfer necessary to activate the brake while moving forward, I can't imagine a backing up scenario that would cause the brakes to be applied on that rig.
Also have a motorhome with a newly built surge brake tow bar for a toad. Same dynamics seem to apply. Despite the fact that backing up with a toad behind a motorhome is discouraged, it can be done when absolutely necessary and without causing the toad brakes to engage.