All water-works corporations shall hereafter be solely responsible for the maintenance and replacement of all mains, service pipes and facilities placed within a street, avenue, road or way as hereinbefore defined or easement area used by said water-works corporations for supplying water to its customers; and if adequate maintenance requires the reconstruction or replacement of such mains, service pipes and facilities, said mains, service pipes and facilities shall be reconstructed or replaced by the water-works corporation responsible for maintenance as hereinbefore provided. However, replacement or reconstruction of mains and service pipes less than two inches in diameter in excess of 75 feet in length heretofore installed and privately owned, shall be governed by the provisions of section 501.4 of this Part.