You did not mention which master cylinder or vacuum booster you're using, or the year of the 3100 you're modifying. The original equipment master cylinder WILL NOT work with the brake system you're describing, and if you're mixing front and rear brake components it's entirely possible there won't be a suitable master cylinder available. The diameter of the brake lines is totally irrelevant. It's the diameter and stroke of the master cylinder piston and the amount of fluid displaced to operate the front and rear brake cylinders you should be concerned with. Lots of physics and engineering by the manufacturers goes into designing a safe, effective brake system, and that time and money would not be spent if any old collection of parts would be adequate.
Jerry

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