To predict and minimize squeal propensity in brake systems, numerous models, which perform friction induced vibration, are available. However, today there is no model that can explain satisfactorily the dynamic behavior of brake systems. We argue that this is - among other effects - because existing models are based on decaying or constant coefficient of friction, although many investigations indicate rich dynamics of the coefficient of friction. Stability analysis of linear differential equations with periodic coefficients shows that instability regions change with rising amplitude of the periodic coefficients. If the periodic coefficient corresponds to the coefficient of friction, its periodicity can lower the stability of a minimal model.