In my experience , Surge brakes actuate when backing unless you have a solenoid valve to lock out with 5 pin connector for wiring. Your boat trailer probably has this feature, many car dollies do not.
I have actually had the one bolt that holds a surge brake coupler together break due to dolly wheels getting stopped by debris on interstate highway. I'm glad my break away cables held. One advantage to electric brakes is more than 1 bolt and no sliding connection in the coupler like surge brakes.
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