HYDRAULIC SYSTEM OF TRACTOR

PROBLEM TO BE SOLVED: To solve a problem that a tractor (lawn mower) starts a mowing work, when the grounding rollers and a gauge wheel of an implement (mower) mounted on the tractor is grounded on the surface of the ground and then when the control valve of the hydraulic valve of a lifting unit is operated to a lower position, but can not carry out the mowing work in a precise mowing height, because the implement has a large weight to sink the grounding rollers and the gauge wheel in soil. SOLUTION: A power steering unit 3 for steering front wheels, a lifting or lowering unit 6 for lifting or lowering an implement, and a hydraulic change unit 7 for changing a travel power are allowed to communicate with each other in series through a series of hydraulic circuits. On the basis of the lowering state operation of the hydraulic valve 9 of the lifting or lowering unit 6, the charge pressure of the hydraulic stepless change device 25 of the hydraulic change unit 7 is utilized to impart a back pressure to a hydraulic cylinder 8.