The rapid innovation underway with vehicle brake safety systems leads to extensive evaluation and testing by system developers and regulatory agencies. The ability to evaluate complex heavy truck braking systems is potentially more rapid and economical through hardware-in-the-loop (HiL) simulation which employs the actual electronics and vehicle hardware. Though the initial HiL system development is time consuming and expensive, tests conducted on the completed system do not require track time, fuel, vehicle maintenance, or technician labor for driving or truck configuration changes. Truck and trailer configuration and loading as well as test scenarios can be rapidly adjusted within the vehicle dynamics simulation software to evaluate the performance of automated safety interventions (such as ESC) over a wide range of conditions. Hardware-in-the-loop simulation does not obviate the need for all track testing; vehicle models for simulation must be validated against track data for each truck platform. But HiL simulation can supplement and extend track data for tests at higher speeds, low friction surfaces, and alternate vehicle configurations. A HiL pneumatic braking system was developed for this purpose by the National Highway Traffic Safety Administration, with the goal of evaluating performance as it relates to safety. This paper describes the system in detail and includes some sample results of the testing.