MasterCylinder/Brake question

I have a question about the brake actuating arm that goes into the back side of my master cylinder. I have a 70 Duster with front disc's & Rear drums.

On the end of my actuating arm there's suppose to be a small O-ring that causes the arm to fit tight into the master cylinder.

Where does one find a replacement O- ring and is there any thing I nee dto know as far as installation.

Thanks