I had the same problem on the monolever g/s final drive. Oil was definitely seeping trough the brake cam shaft and onto the drum surface. Even replacing the o-ring with a BMW o-ring, it still leaked. I then took the shaft and machined a new slot for a second o-ring next to the original one.
Worked perfectly until a week later when the leak migrated to the outside, leaking oil down the brake lever onto the rim.
Maybe it could be the vent valve on the top that is clogged or something, forcing the oil past the o-rings when the temperature and pressure increases.