Primary chaincase breather?

[Phil C]

On my 1965 B40F I have recently got rid of the chain oiler, replacing it with a nyloc nut and fibre washer. Julian S on this forum mentioned putting in place a breather. Rupert Ratio (p.125) advises getting rid of the oiler and replacing it with a nut/bolt and fibre washer, but does not at that point say anything about a breather. However on p.165 he says the primary chaincase (in isolation from the engine breathing system) allows expansion and contraction of the air within it via the chain oiler and/or a chaincase inspection cap with a small drilling and steel baffle plate.  So it sounds as though , as Julian suggested, I might need a breather of some kind, now that I have closed off where the oiler was.  Being a novice, I don't really understand the why a breather isn't a  leak!  Could someone (Julian and/or anyone)  please explain and advise ?  Thanks.  Phil.

[AdrianS]

Don't need a breather. Just seal off the oiler!