Trailer braking system for a towing vehicle

A trailer braking system for a towing vehicle, comprising:

• transducer means for generating electrical braking demand signals in response to a demand;
• trailer braking control means adapted to receive said electrical braking demand signals and to produce an electrical control signal which varies with the braking demand;
• means for producing a control pressure in response to the control signal in order to regulate a trailer braking valve whose opening is regulated by said control pressure and which thereby modulates a pressurized air supply to the valve to generate a trailer braking pressure;
• means for supplying a back-up control pressure simultaneously with said control pressure to regulate the opening of said valve;
• and
• means for comparing said control pressure and said back-up control pressure and for enabling actuation of said trailer braking valve by said back-up control pressure when the difference in values between the control pressure and the back-up control pressure indicates insufficient braking by the control pressure.