The Bendix Trailer Vac brake actuation system is designed for trailers equipped with automotive hydraulic brakes. Engine manifold vacuum is the power source and trailer brakes are applied in direct proportion to the tow vehicle braking level. The Trailer Vac system is available in three sizes and is capable of braking trailers weighing from 6,000 lb. to 40,000 lb. Significant improvements in the braking performance of combination vehicles has been demonstrated. The paper describes the features, development, performance and application of these systems.