Abstract: A hydraulic damper for controlling the lag-lead movements of a helicopter rotor blade has dual oil seals on its piston rods. A fluid conduit connects the area between the seals to the area beneath the piston of a cylinder-and-piston type indicator and also through a fluid restriction back into an open fluid system. A bypass conduit is extended from the fluid conduit on the downstream side of the restriction to the area above the indicator piston so that any back pressure in the fluid system which creates a pressure rise of the other side of the restriction also acts on the upper end of the indicator piston.