The article presents the relay valve of braking systems of rail vehicles and its functions. Because of their function in braking systems the relay valves should be characterized by high reliability and correct carrying out the tasks in the braking system of rail vehicles. The stand tests of relay valves have shown that the properties of the applied grease have a big influence on the correctness of the relay valves' functioning. The paper presents the test stand and the results of tests of lubricants used in the relay valves. The idea of carried out tests of the values of lubricants parameters in different temperatures is discussed. The analysis of the test results of the lubricant parameters and the tests of the relay valves on the test stand showed that the correctness of the relay valves operation depends on the properties of the used lubricant. On the basis of the carried out tests, a grease was selected, which especially in low temperatures, meets the criteria of the correct and reliable operation of relay valves in rail vehicles.