The board of directors may provide for a bond reserve fund for the purpose of payment of the principal of or interest on bonds of the district as the same shall become due. Except as otherwise provided in the resolution providing for the issuance of said bonds and so long as any of said bonds are outstanding, any moneys set aside in said bond reserve fund shall be used solely to pay the principal of or interest on said bonds as the same shall become due, in the event and to the extent that other available moneys of the district are insufficient therefor. When all of the principal of or interest on said bonds has been paid, the balance of the money remaining in the bond reserve fund shall be transferred to the general fund.