The security for Alameda-Contra Costa Transit District, Calif.'s bonds requires the district to make monthly deposits under an installment sale agreement from any available monies. There is no pledge of specific revenues. The rating reflects: * Weak financial position, * Decreasing ridership volume, and * Weak legal structure. The weaknesses are offset by good debt service coverage levels. The district covenants to budget installment payments each year, to take all steps legally possible to make monies available, and not to use any revenues in any year to pay operation and maintenance expenses of the system before the monthly installments. The area served by the district, which directly employs about 2,000, participates in the San Francisco Bay area's large employment base