Until all bonds of the Authority, including refunding bonds, and the interest thereon are paid in full, the Commonwealth Transportation Board may use any part of funds available for the maintenance of state highways in the highway construction district in which the Authority’s facilities are wholly or partly located to provide for such portion of the operation, maintenance, and repair of the facilities of the Authority as is deemed in the public interest; however, no part of such funds shall be used for the facilities of the Authority unless the fees, tolls, rents, rates, and other charges for the use thereof are not sufficient to make the required payments of principal and interest on the outstanding revenue bonds issued in connection therewith, and to operate, maintain, and repair the same.