A device and method for providing a warning to an operator of a truck/tractor and trailer unit of a potential loss or reduction of brakes. The invention provides a pressure differential sensor (12) which bridges between an air supply line (13, 16) to the spring brakes (17) of a truck/tractor and an air supply line (14) to the trailer. An audible and/or visual warning device (31, 33) coupled to the sensor (12) is activated upon a differential between the pressures in the respective supply lines (13, 16 and 14) being sensed.