Brake Controller

I would check the voltage first and see what the volage drop is. Then manually apply the brakes and perform the same test. If the voltage was not OK in the first test, the controller or controller power wire, if it passes the first test then fails the second the wire from the controller back are suspect, if you're in the rust belt take your connector appart, remove green stuff, if you can follow the wire have someone hold the trailer brake on while someone moves along the wire feeling for heat be very careful some wires can go from cool to white hot in a nano-second so never put skin on the wire or where space permits a digital thermometer works well

How old is the trailer. The brakes on trailer Could be the problem. Check brake adjustment and magnets make sure there's no rust where magnets grab drums. Also check your truck manual to see if it Has a adjustment for hydraulic disc brakes on trailer.. hope this helps