[dropd80s]

I believe JB1 is manual brakes which should have 1" rotors, JB3 is power brakes with 1" rotors and JB5 is power brakes with 1.25" rotors. I don't think which drums you have will affect which master you choose too much, I think all the rear wheel cylinders are the same any way. I have swapped JB3 brakes on to trucks with JB5 master cylinders and vice versa with no problems. I would just get the booster and master cylinder that matches you front brakes, JB3 for 1" and JB5 for 1.25".