The object of research is a differential mortar pump with a horizontal pump column, designed for pumping construction mortars of various mobility to the places of their mechanized application to the treated surfaces. To its disadvantages should be attributed insufficiently studied mechanism of formation of return leakage of the mortar, which directly depends on the value of the angle of closure of the valves. As methods of research, methods of physical and mathematical modeling, similarity theory, as well as a personal computer for computation and presentation of their results are used. The issues allowing to estimate the efficiency of the differential mortar pump operation with the horizontal arrangement of the pump column for the efficiency of their valve units are considered. The possibility of estimating the operation of valve assemblies according to the angle of their operation is considered. The calculation procedure is based on the analysis of the design of the valve assemblies and the determination of the time of their opening and closing. Separately, both suction and discharge valves are considered from the point of view of the sequence of their opening – closing and minimizing their operating time. The time of their operation is usually determined based on the speed of the mortar passing through the valve assembly. The operating angle is considered as a universal index that allows comparing pumps of different designs with different technological characteristics and allowing to improve their design, achieving a reduction of this index. In addition, the reasons allowing to influence the reduction of the closing time of the valve without degrading the technological characteristics of the mortar pump itself are considered. The diagrams of the mortar pump are constructed, along which the real angles of operation of the valve assemblies are determined. The results of the comparison testify to the correspondence between theoretical (16.8o) and practical (17.3o) data and the possibility of using numerical values of the valve response angle as an indicator of the efficiency of the mortar pump as a whole.