How is weak isospin conserved? Before spontaneous symmetry breaking, it's conserved - by construction. Afterwards, the vacuum expectation value of the Higgs breaks the symmetry. It's the mass terms, and interactions of the Higgs field, where the symmetry is broken, and so there is no conservation for these processes. The mass terms, however, can appear in interaction calculations through propagators and spinors, and so anywhere there is mass should there not be non-conservation of isospin?