What was found
AT FL 350, ALTIMETER WARNING HORN SOUNDED. CREW PERFORMED RAPID DESCENT TO 16,000 FEET. LEFT PNEUMATIC DUCT PRESS INDICATED 45 PSI AND RT, 13 PSI. MAIN OUTFLOW INDICATED FULLY CLOSED AT ALL TIMES. FLIGHT LANDED NORMALLY. REMOVED AND REPLACED PRSOV PNEUMATIC CONTROL TUBE, AND OPS CHECKED PER BOEING M/M.