The specific gravity of a substance(brass in your case) is the ration of its density to the density of water at 4 degrees C, which is 1.0 x 10^3 Kg/m^3. (the size of the kilogram was originally defined to make the density of water 1.0 x 10^3 Kg/m^3 at 4 degrees C). By definition specific gravity is a dimensionless quantity.
Anyways... specific gravity of brass is 8.4-8.7 kg/m^3. Hope that helps