With electric brakes you can test them using a 12volt battery charger. Each magnet (at each wheel) should draw about 3 amps, and that should also lock up the brake on that wheel. Since you said you cut the wires, it should be fairly easy to check each wheel. If there is no current flow, there is a break in the circuit, either the leads, or in the magnet 'puck' itself. The pucks do wear out and have to be replaced occasionally. Chances are if one is worn out, they all are, plan on replacing the whole works.
If the puck draws current ok, then it's a problem with the brakes themselves, either needing adjustment, or worn beyond repair. Again, plan on replacing all of them.
Make sure that you pull the trailer plug out of the vehicle before testing this way, you don't want to backfeed into the truck and cause damage.
While you're in there, plan on checking and repacking the bearings and springs too. The pivot bolts for the springs (at each end) are in bushings, the bushings wear out. (Some manufacturers use plastic bushings, but you can also get a metal one (a type of bronze maybe? I forget). The metal ones aren't much more $$$ either.
Let us know how you make out, and ask if you have any other questions, I'll see what I can do to help.