sorry, when you say master cylinder, you mean the brake cylinder, right? just so i'm clear.
you can replace it with a whole new one similar to the rear brake if you like. i'm sure that someone can help you with pointing you in the right direction.
failing that, call a bike shop and ask them if they have the seal...
you can replace it with a whole new one similar to the rear brake if you like. i'm sure that someone can help you with pointing you in the right direction.
failing that, call a bike shop and ask them if they have the seal...