If it were the ABS, he would feel the pedal pulsating and vibrating under his foot. Surely he would have mentioned this.
You should NOT feel pulsating and vibrating under your foot when braking unless you are on ice or making a *panic* stop. If your truck does that all the time to the point where you feel it is normal enough to not mention, get your ABS fixed.
There are some simple tests to see what is causing the problem:
1. Turn the brake controller down to 0. Now try stopping with just the truck brakes. If it pulls, it's a problem with the truck.
2. Set the trailer brakes where you had them. Now try stopping with just the trailer brakes. If it pulls, it's a problem with the trailer.
If it turns out to be the trailer, jack it up and spin each wheel individually while a helper operates the brake controller. You will likely find that one or more brakes are not working.
