Abstract: In a vehicle such as a tractor or a trailer having fluid-operated spring brakes to which fluid is supplied under pressure for releasing the brakes from a locked condition, security means are provided to prevent unauthorized release of the brakes. The security means comprises a bistable solenoid valve for controlling the supply of fluid to the brakes, energizing circuit means for operating the valve, and limited access switching means operable to set the solenoid valve selectively in an open or a closed condition.

Abstract: The invention provides a trailer brake monitoring system in which foot brake applications for normal braking and hand control braking for selectively applying the trailer brakes are separately monitored by respective monitoring switch circuits. Signals from the monitoring circuits are processed so as to provide a record of the duration and the count of such applications thus providing a record of the manner of brake usage. A solenoid-operated security system which can be locked and unlocked by a key is interconnected with the fluid supply to the trailer brakes.