Advanced hydraulic system rod sealing requirements can be met by using seals made of plastic materials in applications where elastomers do not have adequate life in the service environment. Exploratory seal designs were optimized for advanced applications using machinable polyimide materials. These seals demonstrated equivalent flight hour lives of 12 500 at 350° F and 9875 at 400° to 450° F in advanced hydraulic system simulation. Successful operation was also attained under simulated space shuttle applications; 96 reentry thermal cycles and 1438 hours of vacuum storage. Tests of less expensive molded plastic seals indicated a need for improved materials to provide equivalent performance to the machined seals.