It is assumed that braking system of a modern concept motor vehicle should be optimised with respect to a number of different parameters. There are cases where the brake performance under test conditions are significantly below designed (“nominal”) values. Such deviations are often attributed to the friction material characteristics, but there are also other possible causes such as mechanical losses in the brake itself. Contribution to the allocation of mechanical losses in drum brakes and quantification of their influence on the brake performance and braking force distribution is presented in this article.It was shown that there is a significant influence of mechanical losses on the drum brake performance, especially in unladen vehicles, vehicles equipped with highly sensitive brakes (high brake factor values) and in cases where vehicle is braked on low friction roads. It was shown, also, that the “mechanical losses function” as defined in this paper should be used in considerations rather then widely used “mechanical efficiency coefficient” of a brake.