Make-up valve are provided in the swing circuit to prevent cavitation.When the swing control lever is returned to neutral during swinging,the motor continues to rotate by the inertia of the machine.This causes the moter to act as a pump and pump oil over the overload relief valve.At the same time,a vacuum is created on the opposite side of the motor.The make-up valves prevent cabitation by supplying return oil to the vacuum side of mortor through make-up line.