A.regulate the fluid flow in either direction
B.restrict movement of hydraulic fluid in one direction, but allow free movement in the other direction
C.allow free movement of hydraulic fluid in both directions
D.allow a restricted fluid flow in one direction only

A.adding small amounts of oil to the system
B.the system has been overheated
C.the system has been drained and then filled with new oil
D.the system has been idle for a long period of time