Hydraulic pump-motor

An axial hydraulic pump-motor, in which a cylinder block, having a plurality of cylinder bores formed around a rotational shaft, slides on a valve plate having a high-pressure side port and a low-pressure side port for controlling an amount of reciprocation of a piston in each of the cylinder bores based on a tilt of a swash plate, the hydraulic pump-motor comprising:

• a residual pressure release port provided on the valve plate and configured to communicate with a cylinder bore, from the plurality of cylinder bores, which is on a top dead center side, the residual pressure release port communicating with the cylinder bore until the cylinder bore communicates with the low-pressure side port;
• a residual pressure acquisition portion configured to receive a representative pressure of a residual pressure in the cylinder bore on the top dead center while the cylinder bore on the top dead center side communicates with the low-pressure side or to obtain an estimated value of the residual pressure;
• and
• a directional switching valve configured to switch and block a flow path between the residual pressure release port and a hydraulic oil tank and a flow path between the residual pressure release port and the low-pressure side port based on the representative pressure received by the residual pressure acquisition portion or the estimated value of the residual pressure obtained by the residual pressure acquisition portion.