Monitoring operating parameters of hydraulic pumps in general is one of the solutions to maintain and control the technical condition of the system during operation. In this study, we construct dynamic model of the axial piston pump to investigate effects of the clearance between the pump pistons and the cylinder block on dynamic characteristics. The Runge-Kutta numerical method is used to determine the pump characteristics. The findings indicate that the piston wear has a significant impact on the ripple levels of the pump operating characteristics (pressure, flow rate, power, and leakage flow). Specifically, at a high wear level of 90 µm, the pump characteristics deteriorate significantly, with the leakage flow rate increasing to 10.1% while the output pressure and the power reducing to 80.9% and 72.8%, respectively. Based on the model presented in this paper, a monitoring system for the radial clearance or the wear level of the pump piston-cylinder contact area can be fully developed, and the wear characteristics can be oriented in the process of self-diagnosing the technical condition of pumps.