The development of brake systems for modern vehicles requires the use of simulation tools to deal with the increasing complexity. In the V model development process, Software-in-the-Loop (SiL) and Hardware-in-the-Loop (HiL) methods are commonly used to test and validate brake system components and functions. Due to the higher technology readiness level at the HiL testing stage, there usually is a gap between the results obtained when simulating SiL and HiL. By being aware of the discrepancies between SiL and HiL, conclusions can be drawn for the development of the SiL model. This can help optimize the SiL simulation and reduce the validation gap between SiL and HiL simulation. The paper presents a comparative study of a SiL simulation approach and a HiL wet ESC ECU brake system simulation environment for different brake maneuvers, such as normal braking, emergency braking, and anti-lock braking. The results of the comparison, especially the extent to which the SiL and HiL approaches differ, are discussed in detail. Based on these findings, possible causes for this gap and solutions to minimize it are presented. The paper concludes, that by reducing the validation gap between SiL and HiL simulations, the quality of the SiL simulation is enhanced, and development processes are simplified. This ultimately leads to a reduction of development time and therefore lower cost, while ensuring quality and performance of brake system functions.