Standard brakes cannot stop a 400 km/h truck. Go to Gameplay Settings and slide the Braking Intensity slider all the way to the right.
Is the 400 km/h mod patched? SCS Software has effectively killed the crude torque-multiplier mods of the past.
You must go to a truck dealer or repair shop and install the high-speed engine/transmission provided by the mod. Best Practices for High-Speed Driving Driving at is risky. Follow these tips to prevent constant crashes: