To me, the brake issue seems like it's an electrical issue either with one of the two 7-way connectors or with the wiring to the connectors. The battery shouldn't make any difference whether or not the brakes work. Even if the battery isn't installed the brakes should work fine.
Of course, without the battery you don't have any break away safety so it needs to be there for safety.
Can you feel the brakes activate when you move the manual lever on the brake controller? If not, or it works sometimes, but not others, then it has to be an intermittent connection somewhere between the brake controller in the truck and the brakes themselves. The 7-way connectors being the most common. some trailers also have a plug on the trailer tongue that the other end of the 7-way connector cable plugs in to.
Ray
Ray
