What was found
ON REACHING FL 350, PRESSURE NORMAL DURING CLIMB & CRUISE. SHORTLY AFTER NOTICED CABIN RATE AND CABIN ALTITUDE INCREASING UP TO 8600 AND RISING. SWITCHED TO MANUAL MODE WITH NO RESPONSE IN FULL DOWN POSITION. STARTED DESCENT TO 10,000 FT. CABIN ALTITUDE STILL WAS CLIMBING AND REACHED 10,000 CABIN ALTITUDE. PASSING FL 240 STARTED TO REGAIN MANUAL CONTROL OF PRESSURIZATION SYSTEM. DIVERTED. RUBBER SEAL RING AT GROUND CONNECTION DOOR MISSING. INSTALLED GROUND CONNECTION SEAL IAW 21-22-04. PERFORMED OPS CHECKED IAW 21-31-00-700-808. OPS CHECKED GOOD. NO DEFECTS NOTED.