I don't understand why filling the BB to the vent plug could damage the seal as the BB does not operate under pressure (hence having a vent plug). If the vent plug was removed and a modified vent tube installed going vertically you could in theory increase the volume of oil to cover the workings and still vent.
Bruce
I think the problem is that the existing orange vent plug will vent air but not oil, so if it overfilled the BB cannot vent and oil then leaks from the seal.
