Any need for a proportioning valve for drum/drum brakes or is one integral?

Just checked my Corvair vendor catalogs,they supply a dual master cyl.kit and a poportioning valve is not used.It is required if you have front discs because of the different stopping charicteristics of the two types of brakes.That is balanced out with drum brakes by th wheel cyl. size .The Corvair uses the same M/cyl. as other Chevs. of the same years.