Section 8. In proceedings under sections one or fifteen against a parent, relative to any minor child, it shall not of itself be a defence that the defendant has ceased to have custody or the right to custody of such child on his own acquiescence or by judicial action. The legal duty of the parent or parents to support a minor child shall continue notwithstanding the absence of a court decree ordering them or either of them to pay for the support of said child and notwithstanding any court decree granting custody of such child to another.