Abstract: A bracking system for tractor-trailer combinations wherein the air delivery time to the trailer brakes is substantially reduced. A boost-air assembly having its own air supply is mounted at the forward end of the air delivery system. The assembly includes a fast-acting amplifying relay valve, a closely-coupled air tank and a bracket for mounting the valve and the tank. In the prime embodiment, the assembly is mounted on the tractor proximate the rear thereof to significantly reduce the time required to transmit a control pressure to the trailer brake actuators because (1) less time is required for the control pressure to reach the amplifying valve, (2) the fast response time of the valve, (3) the amplification achieved by the valve and (4) the proximity of the closely-coupled auxiliary air tank. Alternate mounting loctions are also taught for mounting one or more boost-air assemblies for even further lessening air delivery time to the trailer brakes.