Abstract: A counter balance valve (71L) is located between a directional control valve (23) and a hydraulic motor (32L) and provided on the way of a pair of supply/discharge lines (25A, 25B). The counter balance valve (71L) allows a spool (72L) to be displaced axially based on a pressure difference between the supply/discharge lines (25A, 25B). The counter balance valve (71L) includes a communicating passage (73L) for communicating the supply/discharge lines (25A, 25B) when the displacement of the spool (72L) exceeds a predetermined amount (XCM), based on a pressure difference between the supply/discharge lines (25A, 25B). The communicating passage (73L) is provided in the spool (72L) of the counter balance valve (71L).

Abstract: A counter balance valve (71L) is located between a directional control valve (23) and a hydraulic motor (32L) and provided on the way of a pair of supply/discharge lines (25A, 25B). The counter balance valve (71L) allows a spool (72L) to be displaced axially based on a pressure difference between the supply/discharge lines (25A, 25B). The counter balance valve (71L) includes a communicating passage (73L) for communicating the supply/discharge lines (25A, 25B) when the displacement of the spool (72L) exceeds a predetermined amount (XCM), based on a pressure difference between the supply/discharge lines (25A, 25B). The communicating passage (73L) is provided in the spool (72L) of the counter balance valve (71L).

Abstract: A sliding layer (21) of a sintered copper alloy is formed on a sliding surface (12A) of a valve plate (12). That is, the valve plate (12) is formed of iron-based materials such as cast iron or steel. Then, the sliding surface (12A) which is the front side of the valve plate (12) is covered with the sliding layer (21) formed of a sintered copper alloy containing Cu (copper) and Sn (tin) as principal components and the remaining components as the remainder. The remainder has 2 to 6 wt % CaF2 (calcium fluoride) as an essential component, and an average particle size of said CaF2 is controlled to fall within the range from 40 ?m to 350 ?m. A sliding surface (8A) of a cylinder block (8) which is a opposing part sliding surface is not covered with the copper-alloy sliding layer (21) but is covered with a sliding layer of iron-and-steel based materials.