A technique of calculating the relationships between the discharge pressures in the pumping cavity and the compressor cavity of a hybrid energy-converting piston machine is proposed. Through the use of the results presented here, a relationship is found between the discharge pressure of the pumping cavity and that of the compressor cavity that assures equality between the mass flow rates of the liquid traveling out of the pumping and compressor cavities and, correspondingly, normal operation of a hybrid energy-converting piston machine.