Abstract: A pair of supply/drain pipes continuous with an actuator are respectively communicable with a pair of oil sumps respectively through an intermediation of a pair of extended paths. The pair of oil sumps is formed in a switch-spool insertion hole and enter a neutral state or a state of being selectively communicated with any one of respective pressure-oil paths and respective oil-return paths. A non-leak valve is provided over respective connection portions and between the pair of supply/drain pipes and the pair of extended paths. The non-leak valve is opened when an operation of a pump is started, is maintained to be opened during the operation of the pump, and is closed when the pump is out of operation.

Abstract: A pair of supply/drain pipes (Y and Z) continuous with an actuator (2) are respectively communicable with a pair of oil sumps (A, B, C) and (F, G, H) each other respectively through an intermediation of a pair of extended paths (a and b), the pair of oil sumps (A, B, C) and (F, G, H) being formed in a switch-spool insertion hole (7) and entering a neutral state or a state of being selectively communicated with any one of respective pressure-oil paths (S and V) and respective oil-return paths (Q and W). A non-leak valve (12) is provided over respective connection portions (I and J) and (K and L) between the pair of supply/drain pipes (Y and Z) and the pair of extended paths (a and b), the non-leak valve (12) being opened when an operation of a pump (P) is started, being maintained to be opened during the operation of the pump (P), and being closed when the pump (P) is out of operation.

Abstract: The hydraulic circuit of the invention includes first and second directional control valves 13, 15, each shiftable between three positions. Hydraulic circuit extending from the second directional control valve 15 and a second cylinder 7 to a first cylinder 11 is so configured that, when the second directional control valve 15 is shifted to the first position, with the first directional control valve 13 being assigned to the N-position, hydraulic oil from the second cylinder 7 is directed to the first cylinder 11, so as to cause the attachment 9 to be tilted in the forward direction, and when the second directional control valve 15 is shifted to the second position, hydraulic oil expelled from the second cylinder 7 is directed to the first cylinder 11, so as to cause the attachment 9 to be tilted in the rearward direction.