This topic has been mentioned before but there was nothing done with it as it devolved into a pointless argument on how the system works. https://www.beamng.com/threads/trailer-brakes.29124 Trucks have two brake systems, service brakes and emergency brakes. The service brakes apply to both truck and trailer when pushing on the foot pedal. There is also a handle in trucks that allow you to apply brakes to the trailer only. This allows for a lot more control when sliding or under heavy braking. It would be great to see an extra control slot added to apply the trailer brakes only. Also simulated air pressure that can be built and lost while braking would be great to see on large vehicles
