Good ideas...I would add to the amps reply is you may get a full 12v volts at the brake but it may not get enough amps to get a lockup. I bought a used 21k gn triaxle flatdeck trailer. Very weak brakes. Checked all the brakes for full voltage and amps. Found on axle that the lead wire was frayed badly with just a couple of strands remaining allowing 12v but very little amps. Fixed all my grounds and eliminated crimp/clamshell connections. Finally got the brakes to lock with a full load. I did this to all my trailers before putting them in service.