Abstract: An actuator for use in an active mount system for reducing vibration transmission from a vibrating component to a support structure. The actuator is disposed in a housing that is attached to the vibrating component or the support structure. The actuator includes a piston moveably disposed in a casing, and a piston rod assembly that is connected to the other one of the vibrating component or the support structure. A seal assembly, disposed either in a groove in the piston or in a groove in the casing, provides a seal between the piston and casing. The seal assembly includes an annular seal member comprising polytetrafluoroethylene resin, backing rings on each side of the seal member, and an annular compression spring in the groove between one of the backing rings and one edge of the groove.