Hydraulic drive system

A pump-motor unit (35) including a swashplate (95) having a neutral position (FIG. 3) and first and second displaced positions and comprising first (87) and second (89) pressure actuators associated with the swashplate. Displacement of the swashplate is proportional to substantially only the difference in fluid pressure between a high pressure source (41) and a low pressure source (39). The first pressure actuator includes a first control piston (121) having a first effective area (A1), and the second pressure actuator includes a second control piston (127) having a second effective area (A2), the second effective area being at least somewhat greater than the first effective area. If the high and low pressure sources are at substantially the same pressure, the pressure acting on both of the first and second effective areas will displace the swashplate from neutral toward the second displaced position.