I got some statoseal washers from Earls for my banjo fittings but I should be able to use them any where a crush washer is needed. I got some Tom's Washers which are similar to the Statoseal but have a little tighter fit and little more compression, has any one used them?
I used the statoseal washers in the banjo fittings in my brake lines and worked fine. Not used Tom's washers yet.
I would not use a rubber washer on an automotive brake line fitting . I'd stick to metal crush washers , if they don't seal , there's something wrong with your fittings or installation .
Is the rubber impervious to the brake fluid? If the brake fluid won't attack the rubber and it works I would use it. That being said I would feel more comfortable with a metal crush washer.