Abstract: A vehicle security system for immobilizing a vehicle to prevent theft and other unauthorized movement of the vehicle. The system includes a brake blocking valve for selectively preventing fluid communication between the vehicle wheel brake cylinders and the vehicle brake master cylinder. The brake blocking valve is activated after the vehicle is brought to a stop, the service brakes are applied, and either an activation pushbutton is pressed to engage the system while the engine is running, or, alternatively, the system is automatically activated by turning off the ignition switch while foot pressure is maintained on the service brake. In addition to blocking vehicle movement by locking the service brakes in the brake engaged condition, the system also disables the starter by opening the starter circuit, and it disables the ignition coil by opening the ignition circuit, thereby further deterring and preventing unintended vehicle movement.

Abstract: A fluid power circuit providing a selective operation of a trailer brake system, coupleable with a tractor brake system, comprises a pilot operated valve located between tractor and trailer brake system. The pilot operated valve can be put in a fluid flow passing mode from its fluid blocking mode by simultaneous engagement of two tractor brakes. An independent engagement of either of the tractor brakes, which is necessary for sharp turns in the field operation, does not actuate the trailer brake system.