Theft prevention system for trailers with drop yard feature

There is disclosed a theft prevention system for trailers interacting with the customary pneumatic trailer braking system having spring actuated parking brakes that apply when in absence of pressure in an air supply line to the trailer from a tractor-mounted compressed air tank. A normally closed solenoid valve is connected in series with the pressurized air supply line that supplies air to release the trailer parking brakes and a signal receiver adapted to receive an electrical signal from a transmitter in the tractor cab must be caused to actuate the solenoid to open the valve to release the trailer parking brakes. The signal from the transmitter to the receiver may be a dc, ac, or high frequency signal transmitted over the normally provided electrical cable from tractor to trailer or may be a radio transmitted signal wherein of the kind employed for garage door openers. The solenoid valve is preferably provided with a pneumatic latch so that the parking brakes will remain released as long as air pressure is provided to the trailer once an enabling signal is received by the receiver. The signal transmitter is provided with appropriate security to prevent unauthorized use which may be a key switch, a key pad for entry of an identification number, a magnetic strip card reader or other conventional device. The solenoid valve and receiver in the trailer are secured in a locked cabinet and, if provided, a manual by pass valve for the solenoid valve is also located within the cabinet. The receiver and the solenoid valve are powered from the same electrical cable which provides power to the trailer lights and the receiver may be programed to require the lights to be turned on in whole or in part or sequentially to release the brakes.