Hy VEW, firstly your pictures show Bendix style front and rear brakes. The picture of the brake drum with the tape measure shows me a twelve inch brake drum. Your rear brake picture shows a full floating rear axle, what size of truck are you working on? We will need to see a picture of the rear of the rear axle housing to make any suggestions as to what is under there. As to the master cylinder I would never go smaller on the bore than what the engineers had specified for the stock truck. Master cylinders for drum/drum systems are still available from your F.L.A.P.S. unless you want some fancy chrome thing, the only thing they need to know is the bore and what type of mounting your current one has and what the center to center mounting distance on the mounting flange is, hope that helps.
