I drove a million miles with only a single master cylinder and thought nothing of it. I did have some brake failures and had to dodge some things a time or two, but I never had a wreck because of brake failure. If it worries you, possibly you should go completely through your brake system and install a dual system. This is one of the reasons I hesitate suggesting anyone to make these old machines go faster. If you drive one of these old machines, you need to learn to drive the way we drove them 50 years ago. This is also why I prefer straight air. There is a lot less chance for complete failure.