One of key component of hydrostatic close circuit pump or motor is hydrostatic control. The control influences their performance and behavior. That is the reason to pay high attention on the qualitative aspects. One of them is the First Pass Yield (FPY) indicator.The aim of the paper is to show simulation deployment into the problem solving of leakage issue. High leakages caused low FPY indicator at production line tests. A digital twin was fundamental source of useful information. Selection and finding out the most influencing parameters were based on sensitivity analysis and correlation with the test data and production processes. Final parameters have been found out from their combinations itself in relation to the machining. Outcome of the analysis leads to quality improvement of machining. This improvement of production process of hydrostatic controls resulted to increase FPY indicator close to 100% and reduce its variation into almost 0%.