What was found
CABIN ALTITUDE MASTER WARNING IN FLIGHT AT FL280, WITH ONLY RIGHT PACK OPERATING. CABIN ALTITUDE READ 10,000 FT. DECENDED TO FL 200 AND CABIN DECENDED TO 8,000 FT. MODE SELECT WAS ON AUTO 1. COMPLIED WITH FIM 21-30 FAULT TREE 802, AND PERFORMED A LEFT AND RIGHT CABIN PRESSURE CONTROLLER BITE TEST IAW FIM AND AMM 21-31-02-4, NO FAULTS RECORDED OR PRESENT. PERFORMED VISUAL INSPECTION OF THE CABIN PRESSURE OUTFLOW VALVE IAW AMM 21-31-03-2, NO DEFECTS NOTED. PERFORMED AIR SUPPLY LEAK TEST IAW AMM 36-11-00-5, NO OBVIOUS SIGNS OF LEAKS. ACFT OK FOR CONTINUED SERVICE.