(a) Taxes imposed to pay the interest on bonds issued under this chapter and to create a sinking fund for the redemption of those bonds may not exceed:
(1) 25 cents per $100 valuation for courthouse or jail bonds; and
(2) 15 cents per $100 valuation for bridge or road and bridge bonds.
(b) If the principal of and all interest on bonds issued under this chapter are fully paid and a surplus not exceeding $1,000 remains in the sinking fund, the surplus may be used by the county to maintain and repair the courthouse, jail, roads, or bridges of the county, as determined by the commissioners court.