Revenues shall be the sole source of funds pledged by the local agency for repayment of its bonds. Bonds issued under the provisions of this part shall not be deemed to constitute a debt or liability of the local agency or a pledge of the faith and credit of the local agency but shall be payable solely from revenues. The issuance of bonds shall not directly, indirectly, or contingently obligate the legislative body to levy or pledge any form of taxation or to make any appropriation for their payment.