Nearly the entire quarter of all oil is produced in Russia with the help of sucker-rod pumps, and about a third of all wells are equipped by them. Therefore, the work related to increasing the time to failure and expanding the scope of this type of the equipment use is quite relevant. Based on the statistics of sucker-rod pumps failures, the most common are the following ones: rod breakage and lapel, leakage of the plunger–cylinder pair and malfunction of valve assemblies. Therefore, the correct choice of the design and material of the valve assembly is not an easy task, the solution of which depends on the operating conditions. Since more than 95 % of all sucker-rod pumps in Russia are equipped with valve assemblies consisting of a ball and a seat, a complete understanding and detailed presentation of the principle of such valve pairs operation is necessary under the following complicating conditions: more than 40° of the angle deviation from the vertical in the pump installation area and the production of high-viscous oils. The influence of the ball–seat pair material on the efficiency of the valve assemblies of sucker-rod pumps was studied earlier, and the results of these studies are widely used by machine builders and oilmen in their work. Bench tests were carried out at the Department of Machinery and Equipment for the Oil and Gas Industry of the National University of Oil and Gas "Gubkin University" to determine the scope of using ball valve assemblies under different angles of their deviation from the vertical, as well as at different viscosities of the model liquid. The results of the data obtained allow taking a somewhat broader look at the issues related to the possibility of choosing special designs of valve assemblies under complicated operating conditions of sucker-rod pumps in a well.