My truck was doing that also. Slightly noticeable unloaded but VERY noticeable with the trailer. I knew it was my truck because it did it with different trailers. I've replaced the front brake lines and so far I haven't felt it (knock on wood).
Brake pedal feels MUCH better too my the way!
2018 Ram 3500 DRW CCLB Aisin 4.10 4x4
2018 Jayco Talon 413T
B&W Companion