Working on replacing brake hard lines at the proportion valve and due to rust at the rear brake outlet I snapped the rear brake line. BTW I knew that I was replacing the line anyway. When I removed the brake nut at the rear brake outlet the large nut also came out since the brake nut was so rusted on. Now my question is when I removed the large nut there was a rubber seal between the large nut and proportion valve. If you see in the image below, at the outlet to rear brake location you see a black image on top and bottom which is the rubber seal. Where can I get this or should I replace the whole valve?