Braking system for controlling the braking of a trailer

Braking system on a tractor for controlling the braking of a trailer in a vehicle including said tractor and trailer, said system on the tractor comprising:

• a pressure medium source;
• a driver brake valve having an output and being connected with the pressure medium source;
• at least one brake circuit connecting the output of the driver brake valve to brake cylinders of the tractor;
• a select high trailer brake valve which connects the higher pressure on one of its first and second inputs with its output, said output being connected with brake cylinders of the trailer;
• several sensors;
• an electronic brake system connected with said sensors and which determines the load condition of the trailer, generates a first electric signal which is proportional to the brake pressure to be controlled at the trailer as a function of this load condition, and generates a second electric signal;
• a proportional valve, having an output connected as an input to the trailer brake valves being controlled by the first electric signal, being connected to the pressure medium source, and which provides at its output an output pressure proportional to the first electric signal;
• and
• at least one fail-safe valve having an input connected with an output of the driver brake valve and having an output connected with an input of the trailer brake valve, and the at least one fail-safe valve being controlled by said second electric signal to disconnect output of the brake control valve from the trailer brake valve during the presence of said second electrical signal and to connect the output of the driver brake valve to the trailer brake valve in the absence of said second electrical signal.