What was found
BOI - AIRCRAFT MADE AN UNSCHEDULED LANDING WHEN THE NR 1 HYDRAULIC ISOLATION VALVE/ROLL SPOILER HYDRAULIC LIGHTS ILLUMINATED IN FLIGHT. NR 1 HYDRAULIC QUANTITY WAS OBSERVED AT 25 PERCENT AND FALLING. HYDRAULIC QUANTITY WAS NEAR 0 AT TOUCHDOWN. MAINTENANCE INSPECTED THE AIRCRAFT AND REPLACED THE NR 1 CASE DRAIN LINE AFT OF THE FIREWALL IAW -400 MAINTENANCE MANUAL 29-00-00. OPERATIONAL CHECK AND LEAK CHECK WERE GOOD. ROLL SPOILERS OPERATIONAL CHECK WAS NORMAL. NO FURTHER DISCREPANCIES NOTED. AIRCRAFT OK FOR CONTINUED SERVICE.