Structural modifications of the brake components is an evident passive counter-measure directed on elimination of propensity of the brake to a squeal. The assignment of the unstable eigenvalues into the stable part of the complex plane by the structural optimisation of the non-conservative rotor system requires, however, careful investigation of the doublets in the Campbell diagram. Such an investigation substantially illuminates the perspectives and obstacles for the structural optimisation of brake pads with respect to stabilising eigenvalue assignment and provides explicit formulas, which allow one to assign desirable stable and unstable modes of the coupled system selectively.