  Custodial $SU(2)$ breaking due to dynamical fermion masses is studied in a
rather general context and it is shown how some well known limiting cases are
correctly described. The type of ``gap equation'' which can systematically lead
to extra negative contributions to the so--called $\rho$--parameter is
emphasized. Furthermore general model independent features are discussed and it
is shown how electro--weak precision measurements can be sensitive to the
fermion content and/or dynamical features of a given theory.
