Surge brakes

I finally picked up the new boat today and the trailer has surge brakes. Now I have never had them before but know how they work. The salesman said that when I put the truck in reverse it makes it so the brakes on the trailer don’t work so I can back up without issues. The issue is the brakes engage when I put the truck in reverse and I have to use the key to lock them out. Was the salesman right and it’s my truck with the issue or are they working correctly? Tow vehicle is a 99 GMC Sierra 2500 (new style) with the factory 7 pin trailer harness.

Check to see if you have power going to the reverse pin on your trailer plug. Should be the middle pin.

You could also have a taillight bulb out

Thank you. I just checked and the center pin is loose and won’t tighten up. I will have to take the plug apart and see if I can fix it or else I will just get a new plug. Have never had an issue with trailer lights before but I have never had surge brakes where the reverse pin was needed.