Purpose: Critical defects are of great significance when evaluating the production process effectiveness, especially in the automotive branch. It is important to analyse these defects and distinguish the crucial ones. In this article, the authors have identified critical defects in the process of brake master cylinders production using quality management tools, and proposed improvement actions. Design/methodology/approach: The aim of the article was to analyse defects and the reasons for their occurrence, and improve the production process of master cylinder with special attention to critical defects. In order to improve this process, critical flaws had to be identified and eliminated from the entire set, which was the engineering problem to be solved. Then the causes of their occurrence as well as the factors causing process disturbances affecting the final occurrence of the defect were identified, and actions eliminating or limiting their occurrence were proposed. In the project, they were used to analyse the tools and principles, i.e. process diagram, Pareto-Lorenz diagram, cause-effect diagram, and to improve the Poka-Yoke process. Findings: Based on the examined causes of defects, improvements were proposed, i.e. guides for applying the body, a program stopping the piston assembly station to control the oil level, purchase of caps from sealed material and the need to work in long-sleeved work clothing, moving the body type verification camera from the final to first station, pallet spacers to protect against piston damage, change of supplier, especially of body castings. Research limitations/implications: The major limitation of the paper is that it is based on one case of an organization. In the future, it will be necessary to conduct studies in more organizations so as to find out if the same result can be achieved. Originality/value: Until now, no quality management tools have been used to analyse and organize the formation of defects in production processes.