What was found
AT FL330, NR 2 ENGINE START VALVE OPEN LIGHT ILLUMINATED, UNABLE TO MAINTAIN CABIN PRESSURE. DESCENDED TO 8,000 FT, START VALVE LIGHT EXTINGUISHED. ON ANOTHER ATTEMPT TO USE NR 2 BLEED AT FL 290, SAME SCENARIO OCCURED. DESCENDED TO FL 250, ALL OPERATED OK FOR REMAINDER OF FLIGHT. INSPECTED AND REPLACED NR 2 ENGINE START VALVE AND PRESSURE CHECKED PER BOEING M/M.