Hy Lieut DGoss, to start with I would not go smaller in the bore than the original cylinder. If you are not going to mount your new master cylinder directly to a vacuum booster unit, then you will need two hydrovacs, one for the front system, and one for the rear system. Most of the dual systems I remember have the primary system [closest to the pushrod] plumbed to the rear. The drum/drum master cylinders usually have two reservoirs of the same size. Make sure that the supplier knows that the master cylinder will be used for a drum/drum brake system [disc brake master cylinders do not have built in resisdual pressure valves, which are required in a drum system]. Hope that helps.
