electric trailer brakes are ~14v (battery voltage) max. Most decent brake controllers will provide a variable voltage depending on how hard you're stopping, but your emergency break-away switch should be wired to just put the trailer battery straight across the brakes when activated if something goes wrong.
I'm going to guess you have never tested it before. Sometimes if they rust up inside you can't get them back together.
They are $8.76 on Amazon.
From the looks of yours, I wouldn't bother putting it back on, just replace it. You darn sure don't want it to fail if you need it