There seems to be a degree of misconception concerning this, that can lead to puzzlement and worry!
To start with, I find the terms 'Breather' and 'Breathing' rather misleading: they suggest that air must be able to flow into and out of the crankcase, which simply isn't true.
All that is required is a vent, or exit, to prevent a build-up of net internal pressure caused by piston blow-by.
Some engines (not ABC!) are fitted with a timed, or one-way vent, that maintains a slightly negative net or average pressure, to deter oil leaks.
The tiny one-way valve fitted to a Sopwith crankcase hardly affects this issue, with the free-flow possibility round the magneto's drive-shaft, and perhaps also to the rear end of of the crankshaft.
In fact, any "breathing" results in an amount of power-loss, purposelessly pumping air in and out of the crankcase. But air that is compressed within a crankcase by inward piston-strokes acts as a spring, to help the pistons on their outward strokes.
About the only considerations for a vent-pipe are that it should be small, and as high in the crankcase as possible, to minimise loss of oil and mist.
A footnote: I've come across several crankcases in which the tapping for a one-way valve has never been made. And these engines worked just as well as the rest!