The Bendix® BP-1™ Brake Proportioning valves are incorporated into the air brake systems to improve the controllability and reduce the stopping distance of bobtail tractors during braking. BP-1™, front and rear, valves reacting to the lack of pressure in the trailer supply line reduce the braking effort on the rear axles and increase the braking effort on the front axle of tractors operating in the bobtail mode. Treadle application force during bobtail operation resembles treadle application force during normal operation with a connected trailer. The system consists of a front and a rear brake proportioning valve and a TR-3™ valve. Caution: The BP-1™ front and BP-1™ rear valves are similar in appearance but are NOT interchangeable. Note identification tag attached to one of the cover screws ; it will be stamped ‘BP-1 Front’ or ‘BP-1 Rear’. The BP-1™ rear valve can be used without installing a BP-1™ front valve, providing the vehicle does not have a front axle ratio valve. If the vehicle has a front axle ratio valve, the ratio valve must be replaced with a BP-1™ front valve.