What was found
CABIN WOULD NOT HOLD PRESSURIZATION. BLEEDING FROM NR 1 & 3, PNEUMATIC SUPPLY OPEN. NR 1 AND NR 3 PACKS ON. TRIED ALL (4) MODES OF PRESSURIZATION & CABIN CONTINUED CLIMBING WITH ALL PACKS ON & OUTFLOW VALVE CLOSED. CABIN CONTINUED TO CLIMB & REACHED 12,000 CABIN ALTITUDE. CONTROLLER DESCENT INITIATED, DESCENDING THROUGH 16,000, CABIN FINALLY STARTED TO DESCENT. TROUBLESHOT FOR EXCESSIVE PRESSURE LEAK IAW TAFI 21AA, CHECKED ACCESS DOORS FOR DAMAGED SEALS, CARGO DOOR VENT, SKIN FOR DAMAGE, NO DEFECTS NOTED. FOUND LEAK SOURCE AT AVIONICS BAY DOOR, SEAL OUT OF DOOR SEATING. REPOSITIONED EXTERNAL AVIONICS BAY SEAL PROPERLY IAW MM 52-41-02. PERFORMED ADJ/TEST PRESSURE CONTROL SYSTEM, FOUND NORMAL IAW MM 21-30-00.