What was found
CABIN WAS OPERATING NORMALLY IN MANUAL UNTIL 20,000 FT AND THEN BEGAN AN UNCONTROLLED RISE. DESPITE OUR MANUAL IMPUTS TO DRIVE CABIN DOWN IT DID NOT RESPOND. WE THEN REQUESTED A TURN BACK AND A DESCENT. WITH ATC COORDINATION WE WERE ABLE TO GET CONTROL OF THE CABIN IN OUR DESCENT THROUGH 15,000. THROUGH 15,000 THE CABIN ALTITUDE BEGAN DESCENDING FROM ITS PEAK OF 8,700 CABIN PRESS ALTITUDE. WE GOT FULL CONTROL OF THE CABIN AT 10,000 FT. RETURNED WITH NO EMERGENCY DECLARATION REQUIRED. ON INSPECTION, FOUND ADF ANTENNA SEAL LEAKING AIR. REMOVED, CLEANED, REINSTALLED AND RESEALED ADF ANTENNA. PERFORMED FUNCTION CHECK, FUNCTION CHECK GOOD AT THIS TIME ON GROUND, IAW AMM 34-53-00 REV-B2 & 21-30-00.