service trailer brakes

I’ve got a 2010 duramax and every time I start my truck it says service trailer break system. But when I hook my trailer up and start my truck with my foot on the break it comes across the dash saying “trailer connected” and the trailer breaks work fine until I take my foot all the way off the...

I just posted this same thread over on the EFI Live forums, but thought I would post here too so hopefully I can get some answers quicker. Alright this might be a little lengthy, but I want to be thorough explaining everything so I can hopefully figure out whats wrong. I am working with two...