US Patent 7610884 Valve timing control system

A first-side check valve prevents a hydraulic fluid from being discharged out of a retard chamber, and a second-side check valve prevents the hydraulic fluid from being discharged out of an advance chamber. A first-side control valve opens or closes a first-side discharge passage by a pilot pressure received through a retard pilot passage. A second-side control valve opens or closes a second-side discharge passage by the pilot pressure received through an advance pilot passage. Another check valve is disposed in a supply passage between a phase switch valve and a branch point, at which a supply passage is branched.