What was found
AIR TURN BACK WITH 4 TC'S ON, CABIN CLIMB AT 1,000 FT/MIN WHEN AIRCRAFT REACHED 20,000 FEET CARGO DOOR SEAL LEAKING, CABIN WENT TO 11,000 FEET. INSTALLED NEW BACKUP SEAL ON ENTIRE MAIN CARGO DOOR AFTER AIRCRAFT WAS LEAK CHECKED AT BLAST FENCE WITH SMOKE CANISTERS. PRESSURIZED AIRCRAFT IAW MM 21-30. NO LEAKS NOTED ON MAIN CARGO DOOR.