Backing downhill wont be an issue if there is something wrong with your trailer brake system. Its when you back up on the flat, or uphill, thats when they will lock up. Ive had the brake system lock up on me twice. Once on my 2006 CougarFTD trailer and once on my 2009 CougarFTD trailer. With the 06 it was the solenoid, with the 09 it was the plug that goes into the receptacle on my truck. One day it worked fine…the next day it didnt. Got a new plug and all was fine. Certainly it could be a pinched wire, opening and closing the tongue of the trailer, its really easy to crimp a wire if you arent careful.