2015 Utility Dry Van Trailer WHEELS LOCK UP

[TRUCK365]

We have a 2015 Utility dry van trailer that is having brake problems. For some reason when driver applies the brakes the wheels on the last axle lock up for about 2 seconds and release again. It does this only on the driver side rear axle and when the trailer is empty (no problems at all when loaded). We checked s camshafts and they seem fine. Adjusted the brakes, test drove it and it seemed to have solved the problem. Driver goes out on the road for about two weeks and comes back with same problem from beginning. The ABS light on trailer also came on temporarily but there is no ABS light on the last axle that is giving us problems. Went on to replace brakes, and drums on rear axle along with slack adjusters (they were due anyways). Air pressure is fine, no leaks. The axle seal was also replaced about 4 months ago, but I don't see how that could be the cause of the problem. Any ideas? Any other component that I am not seeing?