The brake lever shaft runs through the final drive and all that keeps the oil in is 2 riubber "O" rings. one each side.
The one on the drum side will be the culprit but change both.
Later model had duplicate "O" rings. and later again had a different final drive setup and a tube went all the way through.
The double "O" ring mod is popular on the drum side. Use one of the existing shallow grooves and machine it to fit the ring.